Welcome to the board!
Ramona Mom to 2 great kids and wife to one wonderful hubby since 1990!
Depends on a lot of factors. For example: how mature is the 13-year-old? Can they be trusted only to add/interact with people they know? Are they already involved in teasing/bullying at school? (I wouldn't let either a bullied kid or a known bully have an account - it's just asking for trouble on both ends.) FWIW, my daughter is turning 13 in a couple of weeks and will be allowed to get an account then; she's proven on other sites that she knows how to be responsible online, and she's unlikely to use it for anything but games and normal chatting with her friends. That said, she'll also be required to give me her password so I can log in occasionally and keep an eye on things - just being on her friends list isn't enough because it's easy to set up a filter and block someone out.
I allowed my DS to have one - we had already discussed internet safety, and he had done well with his internet surfing, including telling me when he accidentally stumbled on something that was inappropriate.
Nope, not yet, wait till they're 16 and a little more mature, FACEBOOK gives birth to drama and stupidity in pre teens and the