I have met increasing numbers of people that only use facebook for logins and silly pictures, or that might have deleted it to avoid a psycho ex, or someone's psycho ex, and never gone back. This is to say like Amazon and Ebay became the baseline of home of the oxygen thieves but some still suffer them anyway, and some opt for better things.
As far as facebook is concerned I don't even maintain fake accounts to use for social engineering and fake identities, probably should as the userbase is skewing such that it would be even more useful (the more technologically illiterate crowd/lag now being in full swing) but eh.
Same reason as always for not being on them -- nobody cares what I do so why broadcast it, and given my tendencies... away from brevity it would seem twitter is not a good fit for me.
I am curious to see what some are calling social networks here as well -- I had always considered skype, discord, whatsapp and, albeit to a lesser extent, snapchat as various flavours of the annoyingly fractured and proprietary messenger market (I thought we had learned that lesson with AIM, yahoo and MSN but apparently we need to learn it again). I guess some offer a token join random/theoretically interesting to you stream option but that seems weak at best - any number of services to point people at a chat room existed prior to then. In fact doing a /list on a IRC network is more likely to find me with new people to talk to than generally having a skype account, and nobody is likely to accuse IRC of being a social network.