The objections are always on the basis of what should be done, what could happen, and the (decreasingly) abstract harm it brings to society as a whole.
Grandma just wants to see pictures of the grandkids and hasn't had a police visit or her passport cancelled because of something she's said online yet, and she probably thinks those concerns are nebulous enough to not worry about, or scoffs at them as crazy conspiracy theory.
Maybe the right thing to do is try to convince her it's worth worrying that her grandkids might live in a dystopian surveillance society where a careless word can cost you your social status and career?
1) are not particularly well-informed about issues around Facebook 2) aren't willing to go through the effort to be well-informed
So the real problem we need to face as a society - Facebook is only one instance of it - is how to get more people to take an interest in the important issues around them? How do we make people into competent citizens?
I don't use Facebook as much, but I see the appeal.