I am on FB. A bit like here, I read a lot more than I post.
MrsC is rather more into it than me. When she was out of work a few years ago, a friend (a 'real' one, not an FB one) told her about Catbook, which is an FB app for people who are mad about cats. Out of that we have got to meet some new people with whom we have something in common, even if it's only the cats! We had a lovely day with a couple in Belgium a year or two back, met another 'friend' in London back in the spring--with said friend coming to see us for a weekend in the summer. The Belgian couple are over in the UK this weekend for a cat show and we met up and they ended up coming back here to meet our cats. All good stuff. Obviously you have to be careful when meeting people to start with--the friend we met in London is a PhD student. She'd arranged to meet us when her parents were over. So the 'virtual' can become 'physical'.
MrsC also uses it to keep in touch with her extended family--making contact with some of them for the first time this way.
We also belong to a group which is rather scattered across the country. They've tried using a private forum, and still do for some things, but the traffic was so low it's proved easier to do most of the communication via FB.
So lots of uses.
As to privacy, I assume that anything I post on the net is effectively public domain, so I'm a little careful. I don't mention work (or at least, very very rarely) for example.
I don't expect everybody to use or like it. What does surprise me is the vehemence with which some people express their anti feelings.
S