In a country where Twitter and Facebook are banned, the microblogging website of choice for 200 million Chinese is Weibo, but tightening restrictions on dissent mean that this platform too is on the wrong side of the vast system of control known as "The Great Firewall of China".
Fearful of the spread of Middle East-style protests against authoritarian rule, and of any destabilising influences ahead of a change of leadership next year, Beijing has been asserting itself among online outlets.
Weibo's operator, Sina, a private company, is cracking down on "the spread of false rumours" after the Communist Party told internet companies to tighten control over information online. Sina has set up a channel called "Weibo Refutes Rumours" to spread denials of false information.
There had been speculation for weeks that a crackdown was coming but the first concrete signs came when Beijing municipal Communist Party boss Liu Qi visited Sina last week and urged the company to "stop the spread of false and harmful information" and "create a healthy online media atmosphere".
Weibo subsequently sent notices to users denying two reports posted on the site, including one about the killing of a 19-year-old woman by a man with political influence. It said the accounts of users who originated the reports were temporarily closed. But Sina will have a tough job keeping a grip on web bulletin boards as these give China's general public a rare opportunity to express opinions to a wide audience.
In so many outbreaks of public anger this year – ranging from complaints about Beijing's handling of the Wenzhou high-speed rail crash to the reaction to anti-authoritarian action in the Middle East – the calls for protest have come on Weibo. Its users have been first to report a scandalous spending spree by an employee of the Red Cross Society of China, and they were the first to reveal how an ancient dish was damaged through incompetence at Beijing's Palace Museum.
It is surprising that it took the government so long to act on Weibo. Like most of China's internet companies, Sina is faced with the choice of introducing a form of self-censorship or risk losing its rights to operate in the world's fastest-growing major online market.