I thin k the two are different. Facebook - if he has friended them, they have to accept the friend request. I don't think it is up to you to police how someone else's child uses facebook. If you feel comfortable with doing so, you could mention to you FIL that this could be seen as odd and suggest that he consider friending only his own family and people he knows personally, but unless there is any suggestion he is making inappropriate comments then I think there is such a huge range of different ways to use facebook it is hard to say he is 'wrong'
Twitter - again, if you don't want to interact with someone you can block them. If a user has not 'locked' their accounts then their tweets can be seen by anyone, you don't control who choses to read what you say.
I think having a general conversation with him, and flagging up concerns about unpleasant conclusions people may draw, but unless he is sending inappropriate messages to them I don't think it is anyone else's business.
Again, lots of ways to use twitter - I think it is quite common to follow people you may to know personally, if they chat to people you do know, for instance. And it isn't always obvious how old or what gender someone is when you follow them.