A. I don't care what 95% of people I am connected with are doing, and I regularly talk to the people I care about anyway.
B. I can't have any meaningful discussions on them
C. They are dominated by high volume posters who tend to be very opinionated. Very opinionated people tend to have extreme opinions which are usually wrong and aren't particularly interesting to me.
D. The networks present a false sense of reality which lowers happiness. Everyone always posts their highlight reel which makes you feel like the whole world is killing it constantly and you aren't.