I've never used mainstream social media (I suppose DW, as a forum, is under that type of umbrella but not really "social media"). A few people at work, managers and whatnot, love FB etc. Some of us vow never to use it.
With the increase of Big Data, or should that be "Bid Data" as it is usually sold? Anyone's data is valuable to someone. Your medical conditions, savings, spending habits are of interest to actuaries and all sorts.
Myself and a colleague compared our investments, accounts and holdings and concluded that Natwest had sold our details as we were getting similar invitations to stately homes for "investment seminars" and that sort of shit. He had recently put a large sum in a current account, three days later a nice heavy-paper, with watermark, invitation arrived.
One big problem (for me anyway) is that even if you're not signed up to Farcebook yourself, they still have a (shadow) profile of you.
I do use FB (sparingly) and I also advertise on it. To my surprise, someone whom I hardly know (and like even less) cropped up on my "People you might know". No common friends, we live 100+ miles apart.... the connection? I'd saved his phone number in my (Android) phone. Now I would never willingly offer up my contacts to a third party, but that's not to say no one else would. Including Google? Or him? Who knows? FB are very coy about how they acquire and use private data, > HERE < is an article I read earlier today.
I also use a browser extension called FBPurity which does a great job of hiding most of the sh*te (ad's, games, 'trending' etc. etc.) you don't want to see.