I am trying to imagine social media in a world without the internet as it exists today.
One option that occurs to me, assymetric cryotography as a substitution for user accounts and as a method for following.
The network is just a giant stream of encrypted posts, all signed with private keys. You have the public keys of the people you follow. Those posts get decrypted and displayed, the rest are discarded.
The UI does all the heavy lifting.