99.9% of everything everyone is afraid of on facebook can just as easily happen via cell phones, email and through plain, old fashioned social contact. The drama, the mean girls, bullying; it went on for years without facebook and will continue long after facebook is a thing of the past.
The only way to keep all of this away from your child is to be a constant part of their lives and to TALK to them. If I didn't listen to dd on a constant basis, I wouldn't know that ALL the girls have boyfriends and that she is being pressured to have one too. I wouldn't know that the class bully is targeting her at the moment and help to give her tools to deal with it (along with talking to the teachers). I wouldn't know the issues she is having with her bff and their little group and that some days when she seems a bit down and out its because of some little tiff. NONE of this happened over facebook, all of it is at school, on the phone, and at softball practice and games and in show choir.
My point to all this is that facebook is a tool, just like email, texting and plain old socializing. Through facebook all the preteen/teen drama can happen but its just happening in a different format and if it is happening on facebook its just a continuation to what is going on in school.
Our kids need to know how to navigate through all this drama. DD needed the confidence to tell her bf that she "doesn't NEED anything" when told she "needed" a boy friend (to which I said "YOU GO GIRL!

). As for the bullying they need to have the strength not to join in and the confidence to just unfriend anyone who starts up with them and then to tell an adult that the bullying is happening. But they need all these tools with or without facebook and we can't look at facebook as the only way these things are happening otherwise we are just fooling ourselves.
Sure kids may post something that isn't too appropriate, but if the kid will post it most of the time they will say it. Why is posting it on facebook worse than saying it face to face?
Someone choosing not to allow their child under 13 to have a facebook account because of the rule, then I understand that. But if you are doing it because of some false sense of security then you really need to think again because facebook does not cause the preteen issues and not having it won't solve it.