I am only on facebook because it allows me to keep in touch with family that is very spread out geographically and because I am a sucker for cute animal videos. :) However, I rarely post anything personal, only posting some of my better hummingbird/wildlife photos (maybe 20 per year), baking pics if I make something unusual like a cheesecake, and a few vacation pictures. Also, I absolutely refuse to forward or otherwise participate in those "post this to your status", or "answer these questions" type of posts. And I don't friend any celebrities, with the exception of Mike Rowe, because I like his posts and videos.
I will say that facebook was an amazing tool when we had the wildfires here and a lot of people were able to connect and help each other using facebook that would have been impossible to coordinate otherwise. Our community set up a page/group for people to use to offer help, request help moving horses and other wildlife, get help to older people, etc., it worked amazingly well and I think I would always keep facebook, just for that feature alone.
I am not on twitter or instagram, etc., because I guess I am too old to get it. Neither of those apps seem useful or interesting to me.