Weibo was used to report the two sessions on March 4, 2011. Photo: CFP

Sina.com's Weibo microblogging service has grown massively in the last two years, reaching over 250 million users. But some liberal users are quitting the service, disappointed by what they see as bursts of irrational debate and content control.

Zhang Ming, a political science professor at Renmin University of China known for his controversial views, left Weibo early last month.

Within days, Yu Jianrong, a prominent sociologist at the Chinese Academy of Social Sciences who initiated a famous campaign on Weibo to rescue kidnapped children, announced that he was quitting Sina in support of Zhang.

He Weifang, a prominent law professor at Peking University, also announced to temporarily suspend his Sina Weibo in mid-January.

Confusing standards

Much of Weibo's influence has come from being seen as a vehicle for more freedom of expression, and as a platform for online campaigns such as Yu's efforts to save abducted kids and the provision of free lunches for schoolchildren. Weibo has seen an explosion of liberal opinions and many taboos being broken down.

However, Zhang said that his posts were deleted, his account was banned from posting, and his followers were removed.

"I can understand that they have to censor some posts. That's fine," said Zhang. "But they shouldn't punish users, not by the order of authorities."

Back in July, Song Shinan, a freelancer and commentator, also wrote on his blog that he's leaving Sina, though his account is still active now.

Song wrote in his blog that he had been censored by Sina for reposting a piece of information from Xinhua. He said he was told not to criticize Sina or post sensitive information.

"What I don't understand is why the four main Weibo services (Sina, Sohu, Netease and Tencent) have such different standards of what can and cannot be posted," commented Zhan Jiang, a journalism professor at Beijing Foreign Studies University.

"It must be a difference in management and self-regulation," he said. "Some probably implement stricter rules out of business concerns."

Western social networks like Facebook remove posts that violate the law or the rights of others. Twitter has also recently announced that it will allow country-specific censorship.

Many liberals remain

Zhang said his complaints to Sina about slanderous rumors on Weibo that he had got a student pregnant were repeatedly ignored, despite going through Sina's own stated procedures for refuting rumors.

Zhang has not been contacted by anyone from Sina since he left. He believes he has got into Weibo's doghouse for his hypercritical comments.

"The goal of Sina Weibo is to create a playground for the rich and celebrities, with some wumao (government stooges) chattering on the side," Zhang wrote on his own account. "Such a playground has no life at all."

However, many prominent users who often take a critical view of the authorities, like Zhan Jiang and writer Murong Xuecun, remain on Sina Weibo. It is still thought to be dominated by liberal voices and some even criticize a lack of conservative opinions as having affected its reputation as the most vibrant forum of public opinions.

Both Zhang and Yu have since moved to Sohu. Some users have written on Sohu Weibo that they've come because of Zhang and Yu. 

Zhang has close to 280,000 followers on Sina and almost 500,000 on Sohu. Yu has over 1.3 million followers on Sina and close to 400,000 on Sohu.