My DD got hers when she was 14, but I supervise it pretty carefully. This means I have her login details, I periodically check her privacy settings, and have been known to do unannounced spot checks. She's a pretty sensible kid though, and we talk about the risks etc. however, we have also taken the precaution of using an alias.

@lisa, it sounds like your dd could tighten up her privacy settings. There's one in there re who can request to friend you.