"The thing I keep thinking is that I want to have social software built on a co-operative model (like a food co-op): you all pay some nominal membership fee (could you fund servers and developers on $5/year/user?), and the software would then be developed to fit the agreed-upon needs of the community"

No fees, no development needed, lightweight and has been working for decades:

A listserv.

Everything else is just candy.