I don't see why Usenet couldn't be used as the backbone for a complete social media platform. Give it a better interface for reading and peer finding (possibly through a re-interpretation of 'friends') that doesn't require setup and you have the perfect multi-machine transport system for posts.
I'd really like to make it happen, but getting a critical mass of people who are willing to set this up and then actually use the damn thing is real hard. Look at the thousands of abandoned phpBB forums scattered across the web, populated with a dozen sub-boards each containing only test messages from the admin. If you don't have people actually using the platform from the very start, the people who set it up first will get bored and stop using it, then anyone new who happens along will see that it's dead and not even bother.
Edit: I'm going to go ahead and put it out there: people who are seriously interested in doing something like this, there's a link to contact info in my HN profile. Email me. A dozen users from HN plus users from the more curmudgeonly online communities I follow may almost be enough to make it happen.
What Mastodon does for decentralized / less-shitty Twitter, this could do for Usenet.