I've just been told about the Briar Project, which seems pretty awesome.
If their approach could be extended with...
● multi-device / multi-node identities
● webserver based nodes for bridging to the classical web - mostly aimed at all those who don't need absolute security
Given those two (admittedly huge) extensions to Briar's model, and some Mastodon-level (or better) UX, I think this could replace facebook nicely.