I have a Myspace, Facebbok and Twitter. Myspace existed before husband left. Still pretty much for what it was, adding bands I like. Both Facebook and Twitter are closed. Only people I accept can see it. Husband's and OW2 FB and Twitter are closed.
I have my SIL on my FB, so I can see their comments on her posts and they can see mine. They can also, if they like, ask her to see my FB. I have nothing husband/marriage/OW related there. Just post things I like, photos, features, music, movie trailers. Not much about my personal life, nor photos of myself or relatives.
Twitter is for professional reasons so the talks there are professional.
I know about husband and OW2 teenage FB romance because of theirs posts on SIL and people that, now and then ask: have you seen what he posted/where he has been has, etc. No, I have not.
Husband as a professional FB. Nothing personal going on there.
Sometimes good things fall apart so better things can fall together. (Marilyn Monroe)