My daughter, I think she was 13 then, started up a facebook account about the time I got one. I didn't want one, but 2 friends wanted me to (now I'm addicted). At that point, my daughter admitted she also had a MySpace account -- without permission. I made her get rid of it.
I think I found out by just saying "oh, I'm thinking of joining facebook" and she has always been open with me and told me then. We are mutual friends on it. She doesn't like me commenting on her stuff (ha...)
I see some of my friends' kids "out there" and I know for sure one set of twins is way underage (they are 9 or 10!) -- they friended me but their mom is on it too. She's our former scout leader!
My daughter seems much more careful than I have been. She even lectures me about friending people I don't know. I usually friend ppl I don't know if they are on a certain prayer page. :)
HTH
Edited: Yikes, just read MzKitty's post. A few months ago, I noticed a funny name on my daughter's wall...turned out her best friend was doing role play with another ID (a la Harry Potter), which led to a discussion with my daughter about that.....