I considered joining Facebook some years ago, but I heard bad things. I sometimes wish I had it, for certain discussion groups or local information. So many things that used to be online are, "Oh we do that on Facebook now."
I'm not on any other social media; they never made sense to me.
Except LinkedIn for business. There, I was put in the position of feeling I had to accept a former client's connection request (I wanted to remain on cordial terms) when I didn't really want to (I didn't know her work, and didn't like what she represented). What was I supposed to say? "Hi, I only connect with people with better credentials, and don't want anyone to think I'm associated with your organization."