It's been used for everything from uniting members with common interests to reuniting long-lost friends and now Facebook users are being asked to help track down a rapist.
The boyfriend of a rape victim in Manchester, England set up a group called "Find the Sale rapist" after a public appeal in the city led to a dead end.
"We just want her attacker caught and I thought starting a Facebook site which would be seen by thousands of people would increase the chances of that happening," the group's creator told the Sunday Mirror.
The page has two images of the attacker captured on closed circuit TV.
Police have asked the boyfriend to take down the page, saying it could jeopardize the case if the man is caught. But, defiant and determined, the boyfriend said, "Someone must know this sick pervert who ruined my girl's life ... I am determined to bring him to justice."
So far, the group has more than 6,175 members.