Early one morning last April, Phượng woke to find over 100 missed calls on her mobile phone, all of them from her eldest daughter, Lý. “Mum!” Lý screamed, when she called back. “Where have you been? Cẩm is missing!” Lý’s voice cracked with panic over her little sister’s disappearance. “She called me from the border and said she’d been tricked! She’s been sold!”

Phượng knew immediately what had happened. They were living in Sapa, an impoverished rural district in Vietnam’s mountainous north-west, and many girls there had disappeared just like this: victims of bride trafficking, destined for China and a life of domestic servitude and sexual slavery. Phượng would have to act quickly if she wanted her daughter back. Traffickers moved at light speed, and 16-year-old Cẩm could very well be in the back of a car already, speeding towards rural China, her “buyer” waiting to introduce her to her new duties: labourer, domestic servant, wife, mother – possibly even sexual plaything for the other men in the family.

Phượng and Cẩm had been having tea together just the day before, and now Phượng remembered that Cẩm had asked to use her phone, to message someone on Facebook. She had said a friend was coming to pick her up, and that he’d be taking her back to the village. Phượng pressed the blue Facebook button on her phone. There was the face of the young man who had come on his motorcycle to pick up her daughter, only to sell her a few hours later across the border.

She ran to the printer’s shop at the top of the road and made multiple photocopies of his profile photo. And then she jumped on to her motorbike and set out to find him, alone.

Cẩm was born in 2000, the third of seven children in an ethnic Black H’mong family. When he wasn’t drinking, her father farmed, and her mother dyed and embroidered the family’s traditional indigo dress: thick, pleated skirts or trousers, paired with long jackets, colourful belts and heavy silver necklaces. Children would look after one another while the adults tilled the rice paddies, or sold clothes at the local market.

Cẩm always stood out. By the age of 12, she had taught herself enough English to befriend the tourists trekking past her family home towards the rice paddies and water buffalo. By 14, she’d dropped out of school to help support her family, and had taken a job at a hotel in Sapa town, a former French hill station now crowded with tourists exploring the nearby mountains. By 15, she’d logged on to Facebook for the first time, escaping online to chat with friends and flirt with boys. Cẩm’s mother, who was married by 16, and had seven children by the age of 37, couldn’t read or write, and had an alcoholic husband. Cẩm wanted more than that.

She met Long on Facebook after he messaged her one day, out of the blue. With close to 5,000 Facebook friends, she was used to boys getting in touch. They would send a friend request, then ask where she went to school, or if she had any recommendations for a visitor to Sapa. She saw Facebook as a sort of online dating website; nearly everyone she chatted to was also H’mong, and it allowed everyone to come together, even if they were from harder-to-reach villages.

The H’mong make up a huge, cross-border ethnic group, and live primarily in the mountains stretching from China through Vietnam, Laos and Thailand. Identity stems not necessarily from the nation of their birth, but the ethnicity of their clan – be that Black H’mong, Flower H’mong or White H’mong. Mostly, they still abide by strict traditions regarding marriage; but on Facebook, Cẩm and her friends were free to chat and flirt.

I asked the boys why they had helped the traffickers and they said, ‘I wanted a new iPhone'

Some of the boys Cẩm chatted to online were cute, but Long was different: they could spend whole afternoons messaging each other. Five months passed like this, until they finally met in person on Cẩm’s 16th birthday. They met once again a few weeks later at Tết – Vietnamese new year – and then, just as suddenly as he’d come into her life, her new boyfriend disappeared.

Not long after, Cẩm was at her grandmother’s house with her mother, a few villages over from their own, when she logged on to Facebook to see if there was any news from Long. Instead, she had a friend request and message from his younger brother Bình, asking if she was in Sapa. “I’m coming up there and don’t really know my way around, are you free to meet up?” the message read. They’d never met before, but she arranged for him to come and pick her up.

Long’s brother had brought a friend as well, and they stopped to collect Cẩm’s best friend, too. The four sat down at a cafe and, as the boys ordered beers for everyone, the girls went to the toilet together to freshen up. Cẩm and her friend went back out to meet the boys and started chatting, sipping their beers as they talked. All Cẩm remembers next is getting sandwiched between the two boys on to a motorbike, feeling so dizzy she could barely keep her eyes open.

Facebook Twitter Pinterest Sapa, on the border of Vietnam and China, is a centre for trafficking. Photograph: Abbie Trayler-Smith/The Guardian

It was the bouncing that finally woke her up: they were hurtling down a lone dirt road in a thick forest. A street sign in Chinese characters came into view, and the bike stopped. Panicking, she dug into her pockets for her mobile phone and started screaming as fast as she could to her sister that she’d been trafficked, she was somewhere on the border with China. An older man emerged from the trees and grabbed the phone. Then he put a knife to Cẩm’s throat. “You’re already in China – you can’t go home now,” he snarled. “I hope you’re ready to get married, because that’s what you’re going to be.”

Human trafficking has a history in Sapa, where thick jungle, slate-coloured rivers and mountainous terrain have allowed people to disappear quickly and at random for years. Men often cross the porous frontier in search of money, hoping to trade goods or become labourers in Chinese mines or quarries. But if girls cross the border, it’s usually because they’ve been snatched: stolen from an outdoor market, kidnapped from the side of the street, even sold by a neighbour or distant relative.

By 2020, there will be an estimated 30 million more Chinese men than women of marrying age

Girls are prized commodities in this part of the world, thanks to the one-child policy introduced in China in 1979. A Confucian preference for male children has resulted in many families opting for sex-selective abortions or performing female infanticide at home. Today, as a result, the nation has what its own officials term “the most serious and prolonged” gender imbalance in the world. By 2020, there will be an estimated 30 million more Chinese men than women of marrying age.

In particularly poor or remote areas, many Chinese men have countered this handicap by “buying” a bride from abroad, a practice billed as a cost-effective alternative to paying a dowry for a local one. The result has seen a surge of women and girls from neighbouring countries peddled into this lucrative bride industry, where “wives” can be purchased for as much as 80,000 yuan (£9,300). Some 1,281 women were rescued and repatriated by Chinese officials in 2012 alone, nearly all of them from neighbouring Laos, Myanmar and Vietnam. The practice sparked international fury at the end of last year, when a 12-year-old Vietnamese girl, who had been sold to a 35-year-old man in central China for 30,000 yuan (£3,400), presented at Xuzhou hospital for her first prenatal appointment.

Last year, Chinese authorities rescued 207 Vietnamese women, and one child, from a cross-border trafficking ring involving 61 gangs. Yet bride trafficking is a phenomenon that authorities struggle to contain. Lào Cai province, which sprawls from these fog-enveloped mountains and grassy valleys to border Yunnan in China, is one of the poorest and most ethnically diverse regions in Vietnam. Home to various hill tribes, including the H’mong, Dao and Tay, the province is popular with backpackers. But monthly per capita income here is just half of the national average, education levels remain frighteningly low and jobs are scarce. In Sapa, a slapdash town of touts and bars that serves as the main tourist base for the region, small children in traditional H’mong dress kneel at the edge of the dusty streets, selling trinkets, their newborn siblings in embroidered papooses on their backs.

Facebook Twitter Pinterest Thiên, 15, with her father: she escaped from five months in captivity in central China, aged 13. Photograph: Abbie Trayler-Smith/The Guardian

Trafficking is easy money for those who have none. “There are many drivers for trafficking in this region: poor education, poverty, puberty,” begins Nguyễn Tuờng Long. Nguyễn heads Lào Cai’s provincial Department of Social Evils Prevention, a division tasked with – among many other things – preventing human trafficking and supporting victims of it. “The most at-risk are ethnic minority girls, who traditionally get married around 13-17, and who are just starting to think about boys for the first time.”

Traffickers consequently target not just girls but boys as well, to help them lure girls over the border. “They don’t see themselves as traffickers,” Nguyễn says of the boys. “They just think about the money they’ll get if they deliver the girl to the border. They prey on the girls by pretending to court them. They take them away from what’s familiar to them – most of them have never left their village before – and then the girls are lost, vulnerable. I’ve interviewed many of the boys who do this, and I’ve asked them: ‘Why?’ And you know what they say? ‘I wanted a new iPhone.’” He shakes his head in disbelief.

The advent of cheap mobiles has made Vietnam one of the most connected nations in the world. Over half of its 95 million population are on the internet, and Facebook – coupled with YouTube – is the country’s most popular platform.

Facebook’s reach, particularly among teens, has spawned the “fake boyfriend” effect. Freed from the traditional constraints of their culture, hill tribe girls like Cẩm are at once able to see into a world beyond their own village and ostensibly make their own choices, particularly when it comes to love. But that freedom is too often a mirage: the online boyfriend who promises marriage is merely a conduit into a hellish world of domestic and sexual slavery.

'How many of you here have lost a daughter, a sister, a neighbour,' she cries

There are no official numbers regarding the number of girls reported missing, nor how many are rescued, says Nguyễn: the border is too porous to police, budgets are small, and rescues depend on the cooperation of Chinese authorities, which may not always be forthcoming. Eighteen Vietnamese girls were repatriated to Lào Cai province in June alone, he says, but in a year, that figure usually evens out to around 100. More cases will go unrecorded.

Many families send search parties into China, but momentum can stall if a daughter’s whereabouts are unknown. As a result, many mothers become their own de facto sleuths: tracing their daughters’ last steps, collating witness testimony, and selling their jewellery or property to keep the investigation going.

Facebook Twitter Pinterest Sương at Bắc Hà market, where she raises awareness about trafficking. Photograph: Abbie Trayler-Smith/The Guardian

At 8am on a Sunday in June, at Bắc Hà market – so high in the Vietnamese mountains it is eye-level with the clouds – one mother-cum-detective, Sương, 43, has climbed on to a makeshift stage armed with anti-trafficking flyers, a microphone and a PA system. Sương’s daughter, Thiên, was sold over the border two years ago, and it took Sương six months to track her down. Her goal now is to prevent more families from enduring what hers did.

“How many of you here have lost a daughter, a sister, a neighbour?” she cries, her voice rasping from the advocacy she performs each week, supported by the US anti-trafficking NGO Pacific Links Foundation, and Nguyễn’s Department of Social Evils Prevention. People slowly turn to face her. “You need to know what to look out for in order to protect your girls. I knew nothing before this happened to me, I had no idea how to keep my daughter safe. Don’t let your girls out of your sight! Be aware who they’re speaking to, who they’re meeting.”

A crowd swells around Sương to share their own stories: everyone here has lost a sister, cousin, neighbour or daughter to the border.

It’s a cat-and-mouse game: the laws against trafficking get stronger, the traffickers get smarter and more sophisticated

“My neighbour’s daughter disappeared a month ago,” a weathered man in his late 40s says, grabbing leaflets out of Sương’s hand. “There’s been no word from her. I want to read up on this, educate myself about what’s going on.” An older mother of five joins in. “In my village, so many girls have disappeared, but no one knows what’s happened to them. Even my own daughter was tricked last year – her fake boyfriend sold her to China and now she’s stuck there. She even has a baby girl.” “My older sister was trafficked by her fake boyfriend,” a young girl cuts in. “She was 16 and met him on Facebook, and he promised to marry her. She was sold in China a bunch of times. The guy who’s her husband now was the final buyer. They’ve got a five-year-old son together now, so she has to just cam chịu [suck it up and suffer].”

Trembling, a petite 43-year-old Flower H’mong woman in a beaded periwinkle jacket approaches us and says quietly: “The 14th of June marked the one-year anniversary of my daughter’s disappearance. She came to this market and never came home.” She starts sobbing. “I think about her all the time. I’m worried she’s dead, and I feel so sick about it that I just want to die.”

Sương steps in. “What had she been up to before she disappeared? Did she have a boyfriend?”

Facebook Twitter Pinterest A woman at Bắc Hà, whose daughter has been missing for a year. Photograph: Abbie Trayler-Smith/The Guardian

“She was very pretty – all the boys liked her – but she was picky,” the woman replies. “She spent a lot of time on her mobile, and that Saturday she got a message and smiled… and she said, ‘My boyfriend just said he wants to marry me.’ I didn’t even know she had a boyfriend. He said he wanted to introduce her to his parents.”

“Was she using Facebook to message him?” asks Sương.

“I’m not sure,” she says, her hands clasped tightly together. “What’s Facebook?”

Is it possible to police a social media platform that has 48 million users? And if not, is it possible to reach every village and hamlet, in every valley and on every mountain, to warn families of the potential perils of technology, poverty – even friendship?

These are the questions that plague Nguyễn and his social workers, whose budget for outreach is less than $10,000 a year. “We have to partner with local NGOs to fill the gaps, because there’s just not enough money to do prevention, education and victim support,” he says. “I’ve been doing this now for 20 years and it’s a cat-and-mouse game: the laws against trafficking get stronger, and then the traffickers get smarter and more sophisticated. They might use a friend to lure the girl to the border, promising cute shoes, cute clothes or new phones. It’s a much more nuanced way of doing things. Why wouldn’t you go with your friend to the border?”

Sương’s 15-year-old daughter, Thiên, has a difficult time answering that question. Two years ago, at 13, she was sold into China by her own best friend, a neighbour in their small farming village. The girls’ mothers were so close that the friends knew them as “aunty”. But early one morning, the friend coaxed Thiên into sneaking off to Lào Cai city and buying some cheap new clothes. Although the city was only an hour down the windy mountain road, Thiên had never before ventured beyond the village perimeter.

The plan was seemingly a trick hatched by her best friend’s entire family. Thiên was physically piggy-backed across the river into China with the help of her friend’s father, sister and brother, and ended up in the hands of a Chinese H’mong couple, who “purchased” her for 75,000 yuan (£8,700) as a labourer and eventual bride for their son, a physically disabled 30-year-old with learning difficulties.

The next five months were torturous for Thiên: starved, beaten and abused, she was forced to mine a stone quarry and physically disfigured if she fell asleep on the job. With help from her mother and the authorities in Vietnam, she was eventually able to escape – a miracle, given that she had been trafficked all the way to Henan province in central China. She later discovered she had been sold by her friend’s family to the Chinese broker for only 1.2m dong (£40) – just enough to afford them some farming equipment.

Even now, two years on, life is an ordeal. The perpetrators – who live just a few metres up the road – have repeatedly offered financial payouts for Thiên to drop the charges against them, and often stop by Thiên’s house to complain that their family name is being tarnished. Although their offers seem an admission of guilt, the police have been unable to find the person they’ve identified as the main suspect in the crime – the family’s eldest daughter, who has since disappeared – and have chosen not to arrest anyone else.

Thiên, still a star student despite her experience, is bullied by her classmates, one of whom is her former best friend. The stigma of being a “China girl” is so acute that many survivors prefer to stay away from home entirely. Some girls choose to board at a reintegration home run by Pacific Links Foundation down in Lào Cai, where they learn basic life skills and attend high school together.

But Thiên is resolute, and refuses to leave the village she has known since birth. “Being sold so that family could buy farming equipment, it makes me feel like I’m nothing more than a water buffalo,” she says. “I’m smart and I’m going to keep studying, and I’ll study as much as I can so that other girls don’t ever go through the same thing.” She wants to be a journalist; her mother, who has since sold land to build Sapa’s only trafficking support centre, beams with pride.

Facebook Twitter Pinterest Phượng with her daughter Cẩm, who was abducted by a Facebook ‘friend’. Photograph: Abbie Trayler-Smith/The Guardian

With no clear road to justice, Thiên’s family have turned their anger into creativity: Sương speaks about trafficking at schools across Sapa, and has adapted Thiên’s story into a play. Her mother likes to perform as the conspiring friend, and her father often assumes the Chinese broker’s role. Thiên always plays herself.

“Women in H’mong culture aren’t normally this strong or independent,” says Sương of her busy schedule. “I look at Thiên now and I’m so proud of her. She is so brave.”

Education is seen as the only clear way out for the girls who have “dishonoured” their families by falling victim to trafficking. Girls who study more invariably earn more, and there is no better remedy here for diminished status than to return home richer and smarter than everyone else, says advocacy director Mimi Vu of Pacific Links Foundation: “There are very few options in society for these girls after returning from China – about 60% of the traffickers who are arrested in Vietnam were once victims themselves.”

Just five days after she went missing, Cẩm was rescued by her mother on the Chinese border. Armed with the photocopies of Long’s Facebook photo, Phượng found his family home and begged his father to help her. Although he denied his son could have been involved, he soon realised both Long and his brother Bình had fled the country, and helped Phượng retrace what he believed were their final steps before disappearing. Meanwhile, Cẩm was in the process of being sold to a H’mong couple at a hotel overlooking a river, when she realised the river was the Vietnamese border and the buildings the skyline of Lào Cai. Screaming that she demanded to go home, Cẩm was suddenly protected by a Chinese-Vietnamese couple, who gave her their mobile to phone home and helped Phượng arrange a boat to rescue her daughter the next morning. “The woman was so kind,” says Cẩm. “She was H’mong, too, and had been sold herself to the Chinese man, so she understood my situation.”

I’m back at school, and I’m studying to be a tourist guide, and that never would have happened if I hadn’t been tricked

Cẩm arrived back home safe and sound. But the stigma of being “dishonoured” forced her out of her village within three days. She is now at a boarding school in Lào Cai, where she is slowly – and anonymously – rebuilding her life.

“I’m back at school, and I’m studying to be a tourist guide, and that never would have happened if I hadn’t been tricked,” she says, her mobile phone at hand, glowing with Facebook messages. Sitting next to her mother, Cẩm epitomises the modern teenager: ripped skinny jeans, a tight white T-shirt, red lipstick.

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At the end of June, a year after she returned home, Cẩm received a letter from the Lào Cai prosecutor’s office, informing her that three men would be charged in relation to her case. She attended their sentencing: the ringleader had only just been released from jail for trafficking a year earlier. The three were found guilty and sentenced to between 12 and 25 years in prison. Bình, the boy who’d originally tricked her, was not among them.

“I never felt like my relationship with Long would lead to marriage necessarily, but I did like the fact that I had somebody to love,” she says quietly. “And now I think my ‘boyfriend’ was just luring girls like me on Facebook into relationships, only for his brother to traffic over the border.

“I feel safer knowing they’ve been sentenced: it’s not just about me and my friend who were tricked, it’s about all of the girls they trafficked,” she says. “But I’m afraid of what Bình might do in revenge. I just want the police to catch him so I can relax, and have a normal life.”

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