From religious extremists to adolescent bullies, social networking sites deal with people pushing free speech to its limit. Now these websites are fighting back by drawing clear lines between freedom and civility.

Mark Zuckerberg, the chief executive of Facebook, likes to say that his website brings people together, helping to make the world a better place. But Facebook isn't a utopia, and when it comes up short, Dave Willner tries to clean up. The 26-year-old Willner hardly looks like a cop. Yet he and his colleagues on Facebook's 'hate and harassment team' are part of a virtual police squad charged with taking down content that is illegal or violates Facebook's terms of service. That puts them on the front line of the debate over free speech on the internet.

That role came into sharp focus last week as the controversy about WikiLeaks boiled over on the web. Facebook took down a page used by WikiLeaks supporters to organise hacking attacks on the sites of companies including PayPal and MasterCard. Facebook said the page violated the terms of service, which prohibit material that is hateful, threatening, pornographic or incites violence or illegal acts. But it did not remove WikiLeaks's own Facebook pages.

Facebook's decision illustrates the complexities that the company grapples with, on issues as diverse as that controversy, verbal bullying among teenagers, gay-baiting and religious intolerance. With more than 500 million members uploading more than one billion pieces of content a day, the site's role as an arbiter of free speech is likely to become even more pronounced. "Facebook has more power in determining who can speak and who can be heard around the globe than any Supreme Court justice, any king or any president, " said Jeffrey Rosen, a law professor at George Washington University. "It is important that Facebook is exercising its power carefully and protecting more speech rather than less. "

But Facebook rarely pleases everyone. Any piece of content - a photograph, video, page or even a message between two individuals - could offend somebody. Decisions by the company not to remove material related to Holocaust denial or pages critical of Islam and other religions have annoyed advocacy groups and prompted some foreign governments to temporarily block the site.

Some critics say Facebook does not do enough to prevent certain abuses, like bullying, and may put users at risk with lax privacy policies. They also say the company is often too slow to respond to problems. For example, a page lampooning and, in some instances, threatening violence against an 11-year-old girl from Orlando, who had appeared in a music video, was still up last week, months after users reported the page to Facebook.

A Facebook spokesman said the company had left the page up because it did not violate its terms of service, which allow criticism of a public figure. The spokesman said that by appearing in a band's video, the girl had become a public figure.

Facebook says it is constantly working to improve its tools to report abuse and trying to educate users about bullying. And it says it responds as fast as it can to the roughly two million reports of potentially abusive content that its users flag every week. "Our intent is to try to make sure we get to the high-priority, high-risk and high-visibility items most quickly, " said Joe Sullivan, Facebook's chief security officer.

In early October, Willner and his colleagues spent more than a week dealing with one highrisk, highly visible case - rogue citizens of Facebook's world had posted anti-gay messages and threats of violence on a page inviting people to remember Tyler Clementi and other gay teenagers who have committed suicide. The team tracked down the accounts of the offenders and shut them down. Then, using an automated technology to tap Facebook's graph of connections between members, they tracked down more profiles for people, who, as it turned out, had also been posting violent messages. "Most of the hateful content was coming from fake profiles, " said James Mitchell, who is Willner's supervisor and leads the team. He said that because most of these profiles, created by people he called "trolls, " were connected to those of other trolls, Facebook could track down and block an entire network quickly.

Most abuse incidents are not as prominent. On a recent morning, Nick Sullivan, a member of the hate and harassment team, watched as reports of bullying incidents scrolled across his screen. "Emily looks like a brother. " (Deleted) "Grady is with Dave. " (Deleted) "Ronald is the biggest loser. " (Deleted) Although the insults are mild, as attacks on specific people who are not public figures, these violated the terms of service. "There's some crazy stuff out there, " Sullivan said. "But you can do thousands of these in a day. "

Facebook faces thornier challenges when policing activity that is political or illegal, like the controversy over WikiLeaks. Last year, the site declined to take down pages related to 'Everybody Draw Muhammad Day', a protest to defend free speech that surfaced in repudiation of death threats received by two cartoonists who had drawn pictures of the Prophet. A lot of the discussion involved people in Islamic countries debating with people in the West about why the images offended. Facebook's team worked to separate the political discussion from the attacks on specific people or Muslims. "There were people on the page that were crossing the line, but the page itself was not crossing the line, " Mitchell said. Facebook's refusal to shut down the debate caused its site to be blocked in Pakistan and Bangladesh for several days.

Facebook has also sought to walk a delicate line on Holocaust denial. The company has generally refused to block Holocaust denial material, but has worked with human rights groups to take down some content linked to organisations or groups, like the government of Iran, for which Holocaust denial is part of a larger campaign against Jews. "Obviously we disagree with them on Holocaust denial, " said Rabbi Abraham Cooper, associate dean of the Simon Wiesenthal Center.

Willner, who on his own Facebook page describes his political views as "turning swords into plowshares and spears into pruning hooks, " makes for an unlikely enforcer. An archaeology and anthropology major in college, he said that while he loved his job, he did not love watching so much of the underbelly of Facebook. "I handle it by focusing on the fact that what we do matters, " he said.