If I could do it all over again, my kids wouldn't get Facebook until they were at least 16. My eldest kids have it. Ds1 has one and he got it when he was 13 or 14, but he is 18 now so I obviously don't keep an eye on his anymore. My DD1 has a FB and got it at 13. I have the password and monitor her use of it because she's already had a few older men try to add her - men who were unknown to us and some from other countries.
She is only allowed to add family, friends from previous schools and close friends from her current school. I will not allow her to be the kid that adds 5000 random people just to get their friend numbers up. She has family photos etc on there that we don't want seen by just anyone. I also check her security all the time to make sure it is very private.