USA Today's post on teens all over becoming more wary of using and trusting Facebook is a recommended read. After it was unveiled that a man lured a teen to her death (Clouding Around), the teens USA Today spoke to have said they've eithered deleted their accounts or have stopped using Facebook altogether.
The real issue here is Facebook and most social networks do nothing to make sure information they gather are safe-guarded or educate their users just who has access to their information. Especially in Facebook's case, it constantly push the envelope to break down privacy walls, change its policies, or revamp privacy settings to obfuscate and confuse users about how their information is being displayed.
So, yes, teens should be wary about predators online because as far as they're concerned, Facebook is a new vast playground for them. It is sad that it has taken a tragedy for kids to be aware of the dangers social sharing without safeguards. And it isn't just for teens. Facebook is a danger to people of all ages.
Criminal opportunists are just lurking about for the right opportunity to pounce. Legally, Facebook probably did not do anything wrong by allowing people to create fake profiles but ethically, it has a lot to answer for.