Angela Zou hardly writes text messages now. Sitting at her office desk in a plush advertising agency, Zou asks her iPhone where they should go to eat. When it buzzes seconds later, she lifts it to her ear for her colleague's reply. The conversation goes back and forth through these snippets of Mandarin before they decide on the bento-box place for lunch.

Like millions of others across Asia, Zou is using WeChat, a smartphone app developed in China, to send voice messages, snapshots and emoticons to her friends. Now that its walkietalkie-style messages have become ubiquitous, she said typing feels like hard work.

WeChat's popularity has grown dramatically since its launch in 2011. Tencent, the company that developed the app, announced in September that its users had doubled in six months to 200 million. The vast majority are in China, though WeChat is being launched across Asia and already has subscribers in the US and UK.

Historically, it has proved difficult for Chinese internet firms to expand beyond the country. But WeChat is being tipped as the first Chinese social media application with the potential to go global.

As WeChat grows, however, politicians and dissidents are voicing concerns: activists fear that the app's voice-messaging service enables security officials to monitor users' movements in real time. And when the app was launched in Taiwan in October, legislators said they feared that it posed a threat to national security, through the potential exposure of private communications.

WeChat is similar to the popular US-based mobile messaging service WhatsApp, but it does more. An amalgamation of social media tools akin to Twitter, Facebook and Skype, it comes in eight languages including English, Arabic and Russian.

"I used WhatsApp before I came back to China [from studying abroad] and found all my friends were using WeChat," said Zou, who is 25. "Now when I want to contact someone I use WeChat first." The app's features include Look Around (known colloquially as the "hook-up" function), which allows users to chat to strangers nearby, while Moments works like Instagram.

But – like similar social media tools – WeChat can also access users' contact books, text messages and determine location through GPS. In China, some fear this could potentially make targeted users susceptible to surveillance.

Hu Jia, a human rights activist jailed for three years on a charge of sedition, suspects that voicemail messages to his friends had been listened to by guobao officials (internal security bureau).

"I took a chance and assumed WeChat was relatively safe," he said. "It's a new product and not developed by China Mobile or China Unicom, [two of China's main telecoms companies], which have been monitoring my calls and text messages for over 10 years. But the guobao surprised me with their ability to repeat my words or voice messages verbatim, though I'm sure I only sent them to some friends through WeChat."

Tencent, the country's biggest internet company, declined to comment for this article but told the South China Morning Post in November, it said: "We have taken user data protection seriously in our product development and daily operations, and at the same time, like other international peers, we comply with relevant laws in the countries where we have operations."

Adam Segal, a Council on Foreign Relations cyber-security expert, said that WeChat was not alone in offering potential security loopholes. "Information technology services and software are all fundamentally insecure," he said. "WeChat shouldn't be singled out in this instance. Many technologies have some type of vulnerability, and a directed adversary can figure out vulnerabilities to exploit and gather intelligence."

Users worldwide, adds Segal, should remember that though an app may have been created in the US, it is not immune to cyber-attack. "Vulnerability runs deeper than the app, it's in the device itself," he says, adding that HTC handsets and iPhones are likely to have been made in China.

Additional reporting by Xian Kenyun