The trouble is that only the most engaged minority of users would be willing to pay for the service directly. What I'm interested to see is whether we can build services where the people who care most end up covering the costs for the rest of the users. So you pay $5 for a facebook-like service that then provides free services to everyone who knows you. You would have all the same funding problems as other public goods but there are plenty of them that make it work. Of course, even if something like a public radio or other public support model would fund a social network sustainably, you would still have the network effect problem in terms of getting people actually switched over.