These p2p "sharing economy" sites are begging for a solution that's actually p2p. The star-rating of users could be maintained by a modified form of bitcoin. I don't know what the business model would be, but that's pretty much the point.
A p2p distributed ad network would be interesting, if the money can flow back to the server operators maintaining the network. Overall though I just meant there really wouldn't be a profitable business model, much like BitTorrent, and that would be a good thing I think by eliminating a powerful middleman.
