Users of Sina Weibo -- China's answer to Twitter -- sent millions of messages criticising the official response to the disaster, which killed 40 and forced the government to halt the expansion of its high-speed rail programme.

It was credited for prodding China's state-run media into an unusually critical response to the disaster, accusing authorities of ignoring safety concerns in their rush to develop the world's biggest high-speed network.

"Launched less than two years ago, Weibo.com has become an online phenomenon with registered accounts recently surpassing 200 million," Sina.com chief executive Charles Chao said in the company's second-quarter earnings statement.

China -- which has the world's largest online population with 485 million users -- constantly strives to exert its control over the Internet, blocking content it deems politically sensitive as part of a vast censorship system.

But the rise of China's weibos -- microblogs similar to Twitter, which is banned by the communist authorities -- has exposed the difficulty of controlling access to information.

A microblogger living near the accident site in the eastern city of Wenzhou is widely believed to have broken the news of the crash, while millions of others kept up a steady barrage of criticism in the days that followed.

The scale of the response appeared to take authorities by surprise. Shortly after the accident, the People's Daily, the mouthpiece of China's Communist Party, urged officials to use the weibos more to communicate with the public.

Sina Weibo is by far the biggest of the country's social networking sites.

It is China's third most popular website overall after search engine Baidu and QQ.com, an instant messaging service, according to the Internet Society of China.

Sina.com's net income in the second quarter fell to $10 million from $25.2 million in the same period a year earlier. But sales in the period rose 20 percent to $119 million.

The state-run China Central Television (CCTV) earlier this week claimed users of the Nasdaq-listed Baidu were losing money on phoney airline tickets allegedly sold by advertisers on the search engine, among other charges.

CCTV's business channel also aired footage of an undercover journalist receiving coaching from a man who appeared to be a Baidu staff member, on how to get approval for pharmaceutical ads using fake documents.

In a live broadcast late Thursday, Wang Zhan, a vice president of sales for Baidu -- which accounts for more than three quarters of China's Internet search market -- apologised "to users affected by the fraudulent information."

"We are reviewing our sales and approval process to try to plug loopholes as soon as possible," he added in comments reported by newspapers Friday.

This is the second time that Baidu has come in for strong media criticism, after state television blasted its advertising practices in 2008, forcing it to revamp part of its business.

At the time, CCTV criticised Baidu for allowing advertisers to pay for space alongside top search results, without labelling it as an ad.

Analysts say the latest round of criticism could have a hidden agenda, such as government worries over the company's near-monopoly on Internet search in China, or attempts by competitors to take away its market share.

During the television broadcast, Wang pledged Baidu would make more efforts to filter information that was illegal.

When asked if the company would change its business model, he replied the Internet environment was "indeed complex" and Baidu would use technology and staff to eliminate fraudulent information.

Baidu shares dropped 6.7 percent on Thursday, bringing total losses so far this week to more than 14 percent.

The firm's major rival remains global Internet giant Google, which partially moved out of the Chinese market last year amid a public spat with Beijing over censorship.

However, it also has a number of Chinese competitors, including new search engines backed by state media giants like Communist Party mouthpiece the People's Daily and the Xinhua news agency.