Twenty million teenagers under the age of 18 have Facebook accounts. Of that group, 7.5 million are under the age of 13—the minimum age set both by federal law and Facebook itself. These numbers come from survey results announced in June by Consumer Reports. The magazine also found that many of the underage users weren't being supervised by their parents and that 1 million children had been "harassed, threatened, or subjected to other forms of cyberbullying on the site in the past year."
This is one lens through which to view kids' use of Facebook—the lens of risk. But you can also argue that the site is a useful social tool for teens, allowing them to strengthen their existing friendships, connect with kids with common interests who they might not otherwise meet, and express themselves with newfound creativity. Yes, there's opportunity for bullying, but that's not necessarily a product of Facebook or social networking more generally. It's a fact of teenage life, however unfortunate