BEIJING - The day before China's Communist Party published one of its most important policy statements in a decade, a copy of the reform plans was already circulating on Chinese social media.
The unprecedented Nov. 14 leak fuelled China's biggest stock market rally in two months as it spread on microblogs and passed from smartphone to smartphone on WeChat, a three-year-old social messaging app developed by Tencent Holdings Ltd.
WeChat, or Weixin in Chinese, meaning "micromessage", leapt from 121 million global monthly active users at the end of September 2012 to 272 million in just a year. It has quickly become the news source of choice for savvy mobile users in China, where a small army of censors scrub the country's Internet of politically sensitive news and "harmful" speech. "For me WeChat is an essential tool," said Hu Jia, a Beijing-based dissident.
Unlike popular microblogging services such as Sina Corp's Sina Weibo, where messages can reach millions of people in minutes, WeChat allows users to communicate in small, private circles of friends, and send text and voice messages for free - a big part of its success.
"Weibo is like a public square, and Weixin is like your sitting room," said Min Jiang, an associate professor studying China's Internet at the University of North Carolina at Charlotte.
WeChat limits the size of ordinary chatrooms to 40 people, and public pages, which users can subscribe to, can only post one message a day.
That does not mean communications cannot be monitored or censored, but it does give users a way to avoid running afoul of the government's new rules, that hold users accountable for "online rumours" read by 5,000 people or re-posted 500 times.
It may also be good news for China's rulers, because messages do not spread as rapidly as on Weibo's open platform.
"WeChat is less of a potential threat to the authorities than Weibo is," said one of the founders of the anti-censorship site GreatFire.org, who goes by the pseudonym Martin Johnson.
"People mostly use WeChat to exchange messages with people they already know. I still think the censors pay more attention to cleaning up Weibo. Weibo messages have the potential to reach millions of people very fast."
Weibo has been particularly singled-out in the ongoing crackdown on "rumour-mongering" by China's stability-obsessed government, which views public protest as a threat to its authority.
But WeChat has not escaped the government's attention, and its explosive growth means it is attracting more scrutiny than ever from the authorities.
"Online communications and national security has already become a conspicuous problem standing before us," said President Xi Jinping during a speech in November, in which he mentioned WeChat by name.
Social media operators in China are required to help censor content on behalf of the government, which trains their employees for the purpose.
Hu, the dissident, says the police have quoted him back messages he has sent through WeChat.