Some mainland internet users were able to gain access to Facebook, the world's largest online social network, for more than seven hours yesterday.
Having been blocked by the authorities for nearly three years, internet users in Beijing, Shanghai, Guangdong and some cities in central China said they were able to visit the site from early morning to mid-afternoon, some through mobile internet connections.
It has since been blocked again.
'Is the wall down?' one microblogger asked on weibo, the mainland equivalent of Twitter, referring to the extensive online filtering system dubbed the 'great firewall of China' (GFW) which the government uses to monitor and censor online information.
'This could be caused by an update to the GFW or an error in processing,' said Hong Bo, a Beijing-based IT commentator. 'There's a chance that some websites could have been released for a short time when the system was updating.'
Facebook was not accessible everywhere, however, with internet users in Sichuan, Guangxi and Xinjiang reporting that it was still blocked yesterday.
A woman in the Ministry of Industry and Information Technology's media office said the issue was 'too technical' for her to comment on.
Facebook, which has more than 845 million active users worldwide, was blocked on the mainland after rioting in Urumqi, the capital of Xinjiang, in July 2009.
The central government believed that Uygur activists were using the site to post details of their protests.
The Chinese Academy of Social Sciences released a report later saying that Facebook 'was used by a foreign intelligence agency to overthrow the governments of other countries'.
Facebook opened a sales office in Hong Kong early last year, giving the company its second office in Asia after Singapore. The office is strategically located to serve the Hong Kong and Taiwanese markets, and potentially mainland China.
The mainland's 500 million internet users are the largest untapped market for American social networking sites.
Facebook has plans for a US$100 billion initial public offering, possibly in the middle of next month.