Silhouettes of mobile users are seen next to a screen projection of Facebook logo in this picture illustration taken March 28, 2018. REUTERS/Dado Ruvic/Illustration/File Photo

A 16-year-old South Sudanese girl was sold off for marriage to the highest bidder on Facebook in November, raising concerns that the platform has become a “latter-day slave market.”

A businessman from South Sudan outbid four others — which included a senior Sudanese government official — after the girl’s family posted a message on Facebook in October, calling for suitors to bid for her hand in marriage.

The post, which has since been deleted by Facebook, was accompanied by a picture of a tall, expressionless girl standing alongside an unknown man. Facebook says it removed the post as soon as it became aware of it, but by then the girl had already been married off.

Child marriage is illegal in South Sudan, but the practice remains widespread with more than half of all girls married before they are 18, according to Girls Not Brides, a global partnership of civil society organizations. The owners of the now-deleted Facebook post seemed well aware of this reality, telling their prospective buyers: “Competition is perfectly allowed in Dinka/Jieng culture.”

A multimillionaire business tycoon from the country’s capital city of Juba named Kok Alat won the auction, after offering 530 cows, three Land Cruiser V8 cars and $10,000 in exchange for Nyalong Ngong Deng Jalang’s hand in marriage. The final bid prompted local media outlets to dub her the “most expensive bride in Africa.”

“This barbaric use of technology is reminiscent of latter-day slave markets.”

Despite the media attention, Facebook says it only became aware of the issue on November 9, three days after the girl was married and more than two weeks after the day the family first announced its auction on the platform, on Oct. 25.

“This barbaric use of technology is reminiscent of latter-day slave markets,” George Otim, country director of Plan International South Sudan, said in a statement. “That a girl could be sold for marriage on the world’s biggest social networking site in this day and age is beyond belief.”

This is the first known incident of Facebook being used to auction off a teenage bride, according to the children’s rights organization Plan International. And activists are worried that the inflated dowry paid in this case will lead to a flood of similar posts, unless Facebook takes a more aggressive stance in the region.

“It is really concerning because, as it was such a lucrative transaction and it attracted so much attention, we are worried that this could act as an incentive for others to follow suit,” Susannah Birkwood from Plan International told VICE News.

Activists are also concerned by Facebook’s seemingly slow response to the offending post, especially given that the incident prompted regional and international media attention long before Facebook took action.

The call for bids was initially posted on Oct. 25. A Facebook spokesperson told VICE News it first learned of the incident on Nov. 9 and took it down the same day, permanently disabling the account of the person who posted it.

“We feel [Facebook] should have much better vigilance and reporting mechanisms going on.”

But by then Alat had already married his teenage bride in an extravagant ceremony in Juba on Nov. 6, with images of the wedding shared on social media and regional websites — some mocking the teenage girl for looking unhappy.

“We feel [Facebook] should have much better vigilance and reporting mechanisms going on, so they can act quickly and efficiently when there are girls rights violations of this nature on their platforms. It is not good enough to say they were not aware of it,” Birkwood said.

When asked what measures Facebook has in place to prevent such an incident from happening again, the company did not immediately respond.

Facebook has faced mounting criticism over its inability to control how people abuse the platform in developing countries around the world. Indeed, this isn’t the first time it has faced criticism in South Sudan: In November 2016 it was accused by the U.N. of enabling the spread of ethnic propaganda without devoting enough resources to tackle the problem.

In the past, Facebook has sought to deflect criticism by blaming local dialects or less common languages as creating barriers to effective policing of dangerous content, but such excuses don’t apply to Nyalong Ngong Deng Jalang’s marriage — as the post was published in English.

Birkwood added that there were “posts all over the platform, joking about and glorifying the situation, and discussing it.” Though some of these posts have been removed, others remain on the platform.

Facebook has only one office on the African continent, located in South Africa. The company largely relies on local users to report hate speech or incidents like last month’s auction.

“This is shocking and saddening,” Nanjira Sambuli, the digital equality advocacy manager at the Web Foundation, told VICE News. “This is yet another example of how tech can be used to perpetuate harm in various contexts.”