Social maturity is variable but parental guidance can constructively shape and drive it. My son has been on Facebook since 10, I am an active friend and monitor, and we talk about what is being posted including comments and photos, etc. He has to take down anything I overrule, but always in the context of a discussion about “why” so that he develops better judgement, discernment and (hopefully!) taste.
He is also limited to amount of Facebook time, and only after homework or chores are finished and lots of daily exercise and activity has occurred. I don’t see FB as any different than anything else that requires me to live up to my role as an active mother engaged every day in helping my child grow up over time to be a socially responsible, fun and good human being.
It’s much better to know your child’s password and observe them online than to say “No internet for you” and have them sneak around you. And they will sneak around you. My niece joined Facebook when she was eleven, lying and saying her age was thirteen. Rather than report her, and have her simply make another account with a false name that I couldn’t track, I added her and kept an eye on what she did. This is my niece, mind you, not my own child.
Children do what they want. Being honest with them, and giving them logical reasons for your actions, and the actions you expect of them is much more effective than trying to completely control what they do, with no regard to how they feel, while repeating “I’m the adult, I’m right.” In their minds, if what you do makes them unhappy, than you can’t be right. Things like that are going to decrease the amount of respect they have for you, and the amount of credence they give to what you say. Of course, you are the parent. You’re free to screw your child up anyway you like.