Weiboreach examined a random sample of 4,500 microblogs each followed by more than 50,000 people. Among these, the aggregate number of posts fell from roughly 10,500 on January 1 to 7,300 on August 31.

The drop in activity on the mainland's biggest microblogging service comes amid an increase in activity on less popular microblogs. A sample of 100,000 such blogs, each with more than 20,000 followers, showed that the number of daily posts rose by 2 per cent per day on average last month compared with previous months. The aggregate number of posts in this sample increased from roughly 408,400 on January 1 to 481,800 on August 30.

The crackdown on rumour- mongering was clearly aimed at so-called Big Vs; commentators with large-scale followings.

The point was underlined on Monday when the Supreme People's Court and the Supreme People's Procuratorate said Weibo users would be held criminally accountable for critical microblog posts that were viewed more than 5,000 times or shared more than 500 times.

Those most at risk are the so-called Big Vs; Weibo users with mass followings whose identities have been verified by Sina. One such "Big V", American-Chinese financier Charles Xue Biqun was detained in Beijing in the middle of last month on charges of soliciting prostitutes. He was paraded on state television, where viewers were told he had "confessed to contracting a bad habit of soliciting prostitution, while he was living abroad to satisfy his peculiar desires and fetishes".

Big Vs are influential due to their appeal among the mainland's young city-dwellers.

Almost all of Xue's 12 million followers, 98 per cent, are under the age of 34, says a survey by the news website Huxiu. Among the 51 million followers of Lee Kai-fu, China's most prominent online pundit, 38 million are under 24. Roughly half of all online posts by university students are reposts of messages by such internet celebrities, according to a study by the Chinese Academy of Social Sciences published last month.

Weiboreach found that the anti-rumour campaign has been effective. Many internet celebrities have not only scaled down their punditry, but also increasingly censored themselves by deleting contentious old posts.

For instance, 2,479 posts disappeared from the microblog of Xu Xin, a prominent Beijing-based liberal commentator, last month, according to Weiboreach data. Xu, a law professor at the Beijing Institute of Technology, declined to comment on why he deleted the posts.

For some observers, the fall in activity among influential microbloggers is not only a reaction to the crackdown, but also reflects a wider trend of influential users moving away from the platform that has dominated the mainland's online debate over the past few years.

In a separate study in May, Weiboreach found that activity on Sina Weibo had fallen to levels last seen in early 2011, marking a reduction of 30 per cent since the peak in the middle of last year. For that survey, a wider sample was used studying those with more than 10,000 followers.

"We may be reaching a point of saturation in either user take-up or of interest in Weibo," said Jonathan Benney, a postdoctoral fellow at University of Oklahoma's Institute for US-China Issues. He said alternatives such as Tencent's WeChat have been gaining ground. WeChat had 235.8 million registered users in August globally, marking a year-on-year rise of 176.8 per cent.

For Fu King-wa, assistant professor at the University of Hong Kong's Journalism and Media Studies Centre, the anti-rumour clampdown is one of the many measures that have led to Sina Weibo's diminishing relevance, that had sparked a downward trend since the beginning of the year or even earlier.

"I expect the falling momentum of Sina Weibo usage to continue," he said.