The fundamental problem is that all this data needs to be hosted somewhere. P2P systems have the issue of persistence: either posts only stick around as long as the people who posted them keep their server online, which is then a burden on anyone who wants to be active in the community, or everyone shares the responsibility for hosting, and then what happens if someone posts CP? Is it just mirrored across the entire P2P system, and each person has to individually root out the CP or just be okay with hosting CP?
Torrents work because you have to actively join a torrent. But discoverability is handled from the outside, through trackers. Trackers choose what they want to host.
Tor or really I2P are the closest equivalents, but they work because everything is encrypted going through them. It’s a privacy thing. With social media, everything is public by design.