Pony Ma, founder of China's largest internet company, Tencent Flickr/TechCrunch WeChat was supposed to be the first Chinese internet service to become a global phenomenon—the Facebook of China, and just as internationally successful. But now it appears that WeChat is censoring words deemed taboo by the Chinese government—and not just in China, but for users everywhere.

To say that this is a problem for Tencent, WeChat’s owner and China’s biggest internet company, would be an understatement. Tencent recently announced that WeChat had surpassed 300 million users, many of them outside of China. (Facebook passed the 1 billion mark in October.) As well as Chinese, the service is available in English, Indonesian, Portuguese, Thai, Vietnamese and Russian. WeChat’s app runs on all the major smartphone operating systems, including Nokia’s Symbian, which is popular in emerging markets.

Words censored globally for WeChat users include the Chinese name of a newspaper currently battling the Chinese government over censorship. The paper’s English name, Southern Weekend, is not blocked on the app.

Here’s where things get difficult for WeChat: Its international users cannot now escape the fact that, as a service with its headquarters and servers in China, it is subject to the censorious whims of the Chinese government. But not only that: Because WeChat reveals the location of messages sent via the service, dissidents in China are now afraid that China’s security services are using WeChat to locate them in real time.

In October, members of Taiwan’s Democratic Progressive Party greeted the launch of WeChat in Taiwan with accusations that the service is a national security risk, saying that “hackers” (among whom they evidently meant to include the Chinese government) could break into users’ communications, including those of members of the Taiwanese military. The potential for states to spy on people through social media is obvious—wouldn’t the CIA like to have access to Facebook’s complete database, for example—but WeChat’s international self-censorship makes those fears seem all the more real.

For now, users of WeChat inside and outside of China could get around censorship by referring to sensitive subjects in languages other than Chinese. But how long will that last? Whatever happens next, Tencent now has a problem that makes Facebook’s occasional privacy snarl-ups look tepid by comparison. It remains to be seen whether this will affect the service’s growth outside of China, but clearly, it’s going to be a stumbling-block for all Chinese web companies with ambitions that go beyond the mainland.