I gave some more thought to the idea of "A folder full of markdown files plus some signatures in place of user accounts plus a static site generator" in place of traditional social, and the more I think about it the more I like it.
No database, no distribution mechanism, no nothing. Just a folder full of .md files.
Rsync 'em, email 'em, syncthing 'em pass 'em around with Dat or however you want.
Give it a dead simple spec, do all the heavy lifting in various available presentation layers.