I think Warren is right.
From what I have seen, people are assuming that because facebook has made a lot of money, that any social network could.
Before I carry on, I'll admit, I do use facebook, But, I have no particular allegiance to it. If another social network comes along that fits in with what I want better than facebook, I'll happily use it.
Most social networks have two sources of income. Neither are guaranteed.
1) Advertising. This is (obviously) tied to the number of visitors a site gets. The problem is that social networks are a very faddy thing. A social networking site can be the most popular site on the web, but it's always possible that another will come along, and the original will fall out of favour. Look at Bebo. Look at MySpace, both of which have been pretty much killed by Facebook. Fewer visitors means lower income.
2) Data mining. No social network admits to doing this (after all, any that did would look like they are spying on their users, which isn't a good thing for any company), but I'd be surprised if they didn't sell on some of the information they gain from their users. This is potentially a good source of income, at the moment. However, it does seem as though various governments are looking at the legality of this. They could, at any time, decide to ban it.