My soon to be twelve year old set up a facebook account without telling us. He was busted by his uncle in less than 1/2 hour. We reviewed what his friends were doing, and were going to let him have an account, when we found out to have an official account you need to be thirteen...."sorry, buddy, the rules say thirteen". So, with their parents knowledge and consent, most of his friends have facebook accounts and have lied about their ages.
I agree with Nifty, than it is a good technique to follow your children's friends and activites. I have a boy, and he is somewhat more communicative than a rock, but not much. It will be good to have an "in" to his mind.