Is there any good way to harness excess bandwidth?

I got to thinking about this when I was once again disgusted by social networking sites for profit. I seem to recall converting some space into webspace isn’t that hard and if enough people did this you could run social networking for free and probably a lot more efficiently.

You might look at what I’m asking about as a sort of peer2peer browsing. You could have info available only online when your computer is on, or you could join a network that juggles the relevant information. Is there a downside?