Disclaimer: I wrote this article and made this website.
There was some talk of this issue in the recent fediverse inefficiencies thread. I’m hopeful that in the future we’ll have a decentralized solution for file hosting but for now I deeply believe that users should pay for their own file hosting.
If a social network is to take off, it must be accessible from mobile devices behind CGNAT (carrier grade network address translation).
Why?
Because that’s where all the users are. The “social” aspect of a “social” network. Anyone can host a forum but it’s useless without users.
p2p from behind a CGNAT works just fine as long as a single server is accessible and can mediate connections between other peers. Most non-servers are behind some sort of NAT these days.