Our kids love it. They play with photos and the additional faces all day long. They aren't chatting with strangers and doing anything wrong. They're just having a good time with it. So I don't think there is any age limit to it.
I would say if they want to chat using it then you should always know exactly who your kids are in contact with. I have every account password and I go on their accounts all the time. Her FB account is on my phone so it's very easy to log out, log in on her, and scroll through stuff, then log back out. She never even knows when I've been on.
As for other programs, there are some that are known to have more audience...like twitter, anyone can follow you. Same with FB unless you turn that option off. I do that on my own and the kids. Friends can follow me or un-follow my posts but strangers cannot follow me.
It's my job to censor their social media. I do it very stringently.