Cost of maintaining my pictures and videos is tiny, as is cost of maintaining Wikipedia (relative to its users). Simple VPS for $1/month is enough for me, and with economies of scale, I'm sure that it's few cents per user at most. Wikipedia has no problem getting enough donation to continue its work.

I like the idea of P2P social network, but honestly I think that there's no problem to solve in the first place. While distribution of huge video files and music files is a problem and torrents solve it, that's why they are popular. It's not easy to host terabytes of video with thousands of downloads. But it's trivially solved with torrents.