When social networking site www.Tribe.net debuted in 2003, it was a place for wide-open online discussions where no topic was taboo. Then the "Terms of Use Guy" showed up.

A made-up character representing Tribe management, "TOU Guy" began aggressively patrolling the site in December, removing nude photos and sending reprimands to authors of offensive material. His enigmatic silhouette popped up in e-mails threatening to banish people who violated the rules.

TOU Guy made Tribe safe for advertisers who had flocked to the site as it became more popular. But original members felt betrayed, and 1,400 of them rebelled and left to build a new site, www.Free-Association.net, where they can once again discuss anything from magic to transgender issues without restriction.

Social networking Web sites such as Tribe, Friendster, MySpace and Facebook grew by 50 percent last year, according to Nielsen-NetRatings, making them some of the most popular destinations on the Internet. But the rapid success is testing the limits of how the Internet's freewheeling free speech can coexist with its power as a medium for advertising and commerce.

Although many sites started out as grass-roots places where friends could meet and connect for free online, nearly all have been purchased over the past several years by big media companies looking for ways to profit from the millions of members by selling advertisements. Many advertisers are squeamish about the nudity, vulgarity and sex that gather on sites where users can post anything that interests them.

"What advertisers are excited about with this new medium is it's a good way to reach young people and people who are buying their stuff," said Lucian James, president of brand marketing firm Agenda Inc. "By restricting it too much, the danger is they turn it into something like old media. People will flee away from something that looks like an old police model or one that looks like it's being run by parents."

The rules about what a member can post to some of these sites vary greatly, and many sites are experimenting with new methods of enforcement. Some, such as online video site YouTube.com, employ a community-policing approach that relies on members to report offensive material, which the company reviews for removal. Others, such as online video site www.vMix.com, take a more active approach, hiring people to view every photo and word before it is posted.

MySpace.com, the most popular networking site with 75 million users, said it recently has stepped up its efforts to ensure that the content posted by members follows no-nudity and no-pornography rules. The company uses a filtering software that tries to recognize nudity in photos before they are uploaded, and it also employs people to screen some photos and videos and address safety concerns.

Last month, Republican lawmakers filed a bill that would force schools and public libraries to prevent children from accessing any social networking site where minors could be exposed to "obscene and indecent material." Rep. Michael Fitzpatrick, R-Pa., one of the bill's sponsors, said it is an effort by lawmakers who call themselves the "Suburban Caucus" to address the concerns of parents who are alarmed by the high-profile arrests of child predators who use the sites to contact teens.

The bill is unlikely to move ahead, given Congress' busy schedule before the end of the session. Some legal experts think the bill would not be constitutional anyway, because it would inherently limit children's access to the Internet and infringe upon their freedom of speech.

"Kids are being kids on social networks — they're doing exactly the same stuff we did when we were kids. The difference is because they're doing it on this public space, we have a window into their activities," said Lauren Gelman, who has studied lawsuits related to MySpace as associate director of Stanford University Law School's Center for Internet and Society.

Social networking sites "have to get this right in a way that they don't alienate their users but they also have to get it right for their investors and advertisers, so you don't end up with a Honda ad next to a pornographic picture," she said. "They have to balance both of these interests."