> It seems impossible to limit someone's exposure on Facebook,

I don't think anyone is forced to use it. I know several families who have account, yet, they don't share on it or use it for anything that could be embarrassing in the later years.

> especially when any 3rd party will happily post a photo of your family member online.

You should always explain to your friends what your expectation of privacy is. If they don't respect that, they are not your friends anyway.