With Web giants such as Facebook and Twitter blocked by the government here, an entire ecosystem of home-grown companies has flourished with names that are unfamiliar to many outside China.

Tencent, one of the country’s biggest tech firms, is hoping to change that with a product that is already one of the fastest-growing mobile services in the world.

The company’s instant messaging product, WeChat, has amassed more than 300 million users — nearly equivalent to the U.S. population — in less than three years. Tencent says there are over 70 million users across southeast Asia, India and Mexico, with 30 million of those added in just the past three months. WeChat has also expanded into Saudi Arabia recently, and there are plans to open a U.S. office.

But WeChat’s Chinese origins could cause problems for it worldwide, just as Chinese telecoms companies Huawei and ZTE have faced in the United States.

Top Web services enjoy extraordinary access to the kind of user data that is coveted by national security officials. China has long been seen as especially aggressive with cyber-snooping, and recent revelations about American Internet surveillance efforts have heightened global concerns about online privacy.

WeChat has already run into some resistance. India’s intelligence bureau has reportedly proposed a ban on WeChat’s services. Analysts predict a potential backlash in the U.S., too.

“It’s one thing when WeChat dominates in China, but when WeChat becomes popular outside China, suddenly China has this access that only the U.S. had before,” said Christopher Soghoian, senior policy analyst on speech, privacy and technology for the American Civil Liberties Union.

Jeremy Goldkorn, director of Danwei.com and a Chinese media expert, said the political issue of where servers that store user data are physically located is going to become more common. “It seems to me many governments are going to want as much control as possible,” he said.

In mainland China, WeChat is ubiquitous, used by everyone from teenagers and their parents to Tibetan activists. It has been called the Facebook of China, but that comparison fails to convey all the things that WeChat can do.

At its most basic, WeChat functions like a free text-messaging service on your phone. Beyond just texts, users can send short videos or voice messages back and forth, like a walkie-talkie.

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