DO NOT leave your children without guide to Facebook, among all others social networks.
Besides harassments and pedophilia, itself, there are unacceptable and offensive behaviors which must be avoid - from kids, I mean. Parents must be both watchful and attentive about what their descendants post.
Guns, drugs, violence, promiscuous language, lies... There are many bad influences (from anywhere!); unfortunately, some, they simply don't catch. After all, there are still innocents - even if measuring more than us! However, I strongly believe that if they want to use it, they must distinguish good from bad by themselves - at least, a little (try to... Be wise!).
Love, dialogue, explanations, books, reading, newspapers, (good) movies, less videogame and internet - that's what they need to understand in a better way this world and for a responsible usage of medias and technology. Further: this inducement provided by their progenitors, then, with school's orientation - mainly nowadays, with all this bullying stuff.