What we did, when our 13 year old ds asked for one, was to get one for ourselves first, so we could see what it is all about. We used it first for a couple weeks. There are many settings, so you can only allow "friends" to view your information. We also have the password to ds' account. It has gone very well so far. DH and I are really enjoying facebook.
I really think as long as you control the settings for his account and make sure that not just any joe-schmoe can read all his info, then things will be fine.