My 9 year old has an account. We have the security settings such that nobody can see anything (not even her profile picture) unless they are already her "friend" and that people cannot send her friend requests. Her only friends on Facebook are family members.
The reason she has an account is so she can keep up with her cousins (who live in Europe - so Skype is hard because when they're awake, we're asleep and vice versa), her grandparents (who live 2 states away), and aunts and uncles (from all over the place). I love that she can talk to them almost every day, share photos with them, links to youtube videos, etc, even though they live thousands of miles apart!
Also, we allow her to use Facebook only when her homework is done, chores done, etc, and she is on the laptop in a place where we can supervise her. My DH and I are the only ones who know her password (she doesn't know her own password), so she can't get on Facebook if we aren't there (for instance, she couldn't get on there from the computer lab at school, or a friend's house). We are conscious of the fact that the internet can be a scary place, but it's also a useful tool in our society, and it's not going to go away. I think by allowing kids to take these (supervised) steps toward responsible internet use, we can better prepare them for a future that will include social media.