I went off facebook a few years ago and then I went off Twitter. No one harassed me, no one said mean things to me. It was just that it was all so depressing for me. There were very few posts that I actually found useful. For me, Twitter was full of strangers announcing job changes, promoting themselves and promoting their politics. It was all the bad parts of a party conversation without the good parts. Facebook was similar, with friends and acquaintances instead of strangers. It was worse than real life.
After going off facebook I started to email some friends and acquaintances. It worked out better. The volume was less, but most people answered back, and it was a better conversation. I can’t think of needing anything to replace Twitter. I will miss some of the NASA accounts I followed which would send out amazing photos from Mars and whatnot, but I can replace that with https://photojournal.jpl.nasa.gov/beta/ . I am finding that targeted communities, such as topic based forums, serve my purposes better so I’ll be doing that. If the apocalypse happens I’ll probably hear about it from my neighbors.