Danielle Bean writes about the rules that she set down for her daughter, Kateri, when she started her own Facebook profile. I think that they are quite sensible, hence I pass on the link to you, especially parents: Six Rules for Facebook.
Actually the rules are quite sensible for everyone, not just teenagers. Especially:
3. Remember: It's forever.The way I put it to young people is "Do not post anything on the internet that you would not want to be posted on the noticeboard in the school hall."
Yes, you can delete status updates, photos, and even "friends" who turn out to be weirdos, but a well-timed screen shot is all it takes to be your undoing -- with your boss, your kid's school, or your mother-in-law. Even if you maintain the strictest of privacy settings, remember that people love to talk and share, right-click and save. Don't ever share something on Facebook you wouldn't want the entire world to know.
I was interested to read that Danielle's daughter was 16 when this issue came up at home. It's nice to read that. Children wander out into the wild too early here in SE England - often without the kind of wise advice that Kateri has been given.