My DS1 & DD1 both have facebook accounts.
My DD1 lied about her age for it and I was okay with that as it was her way of keeping in touch with friends from where we used to live.
I monitor everything to do with my kids facebook accounts. Both of my kids have me & DH as friends. I don't keep anything from them... if I am venting about them they will see it. If something is said on there that they don't want to see, they don't really have to look.
I have my kids passwords to FB & email. I monitor who they add also. I often do a friend cull for them as sometimes it seems that they have excessive amounts of 'friends' whom they never chat to. I also have a secret facebook account that I use to check on the privacy settings each time facebook decides to change something.