I use Facebook, Twitter, and Instagram. My teenage daughter uses twitter and instagram a lot. She has told her friends, "My mom is on there just to make sure I don't do or say anything stupid." I'm still floored by some of the parents that are not on the same sites as their children because when I see inappropriate posts I always wonder whether I should be contacting their parents or not. I've used those situations as teachable moments for my own kids.
My facebook site is set with pretty strict privacy settings but the one feature I often question whether I should use is the "check in" feature. Its definitely an obvious post that I'm not home and can be found somewhere else...both of those are red flags.
My Instagram is very minimal with no bio, no phone, no identifiers except my name. I must approve each follow request before they can see anything. I told my kids that my rule is, if your friends want to follow me, then I will follow them back. I try not to post too many comments on other's posts but will occasionally "like" a post to let them know I am actually active and paying attention.
I try to only post pictures of me when I'm away from home on either site AFTER I return home.
My twitter page is more a professional site so I tend to tweet/retweet professionally. This is newer for me. Doesn't seem like there are many privacy controls on twitter except to block or mute someone following you. I'm finding I need to have a lot more conversations with my teen about this social media because anyone can follow you. Seems a little more "open".