Administrators of Facebook, the popular social networking site, have been asked to explain why the plug was pulled on a string of politically oriented discussion groups in the past few days, as concern grows over internet censorship. Kelvin Sit Tak-O, who runs a discussion group that opposes the pro-establishment party, the Democratic Alliance for the Betterment and Progress of Hong Kong (DAB), said his group's Facebook page was shut down without notice on Thursday. The group had 84,298 members and was aiming for 100,000. Around the same time, a similar group that had targeted 100,000 members to voice opposition to the pro-democracy party, League of Social Democrats, was also closed. 'Apart from my own group, I've heard that other groups with an anti-DAB message have also been closed. We've written complaint letters, but we've only received standard replies about how [Facebook] is working on this case,' Sit said. The closures could have been triggered by opponents flagging the group as 'abusive' with Facebook administrators, he said. A spokesperson for Facebook was not immediately available for comment. DAB lawmaker, Gary Chan Hak-kan, said he had noticed four anti-League discussion groups were removed recently. 'This is outrageous. Facebook claims that it supports free speech. Why does it practise self-censorship over political content? Except in the case of foul language, I do not see any reason for removing any discussion content,' Chan said. The closures come as debate rages over internet providers' willingness to self-censor as they try to gain more market share in China. Controversial Facebook groups were closed in 2008 in the run-up to the Olympic torch relay passing through Hong Kong, as Beijing grew especially sensitive to issues such as Tibetan self-determination. Christina Chan Hau-man, a student protester who waved a Tibetan flag during the torch relay and used Facebook to rally support, had her account closed days before the event. At the time, she said she was told her page had been closed because of 'persistent misuse of the site'. Politicians yesterday urged Facebook to explain the recent closures, stressing that such discussion on the website encouraged more youth participation in politics. District Councillor Ronald Chan Ngok-pang said: 'If people have concerns [about censorship] then Facebook should provide an explanation. They can't just ignore it.' He noted that the internet was an accepted platform for advancing political discussion. Tanya Chan, the head of the Civic Party's youth division, said Facebook encouraged a free and creative approach to politics. 'As a political party, you would expect some people to like you, and other people to dislike you. But at least everyone can express their opinion.' A government spokesman noted that freedom of expression was protected under the Basic Law. Separately, top transport officials will today conduct an online forum lasting three hours to gauge public opinion on transport policy.