I have vague memories of some apps like this one, but with the additional feature of being peer to peer in a sort of local mesh network.

I have a pretty distinct memory of a social media that did this, so

1. user A would pass by user B and B would get A’s data
2. B passes by user C, and gives user C their data and A’s
3. B makes another post
4. B passes A again and A gets B’s new post and C’s data

I vaguely remember a chat app that did a similar thing but A and C meet up first and extange encryption keys. Middle men like B can’t read the messages between A and C, but the messages slowly propagate over the world by sharing data via connections. Once C gets the message he provides some form of proof that he got it and then that message spreads back, telling devices to delete the encrypted message to save space.

I’m positive the first exists, I’m not positive about the second (as I wrote it out I started to doubt myself)

If the second one exists, that would be so incredible. People could setup relay stations for fast communication worldwide while our networks work, but in an apocalypse situation the gears don’t fully grind to a halt