We have a 14 yr old daughter and yes she has one and she is very involved with it.
Our key is that I have the signon name and password. I will surprise check it at least weekly and monitor pictures, and it MUST be set to private profile. We are super cautious with pictures because she is involved with cheer and other school organizations and it is a big no no to have a school logo on the site. I have shown her how pictures are copied and pasted so she is very cautious about what she has on her site.
Most of her friends have it as well. Like most of her friends, she has a laptop with free access. I will check on her computer occasionally. She runs with a good group of kids and until we have a reason to police them more heavily than we already do, we trust her to use good judgement.
We are probably one of the more lenient parents around. We give her space, we keep lines of communication wide open and she knows nothing is off limits as to anything she wants to talk about. We don't monitor keystrokes or sites and won't UNLESS she breaks the rules of the house.