It seems really obvious that sites like Facebook generate revenue from Ads. Twitter doesn't seems to have any revenue model at the time being (CMIIW). So other than advertising what is the revenue channels for social networking sites? Because it seems that from people's responses, advertising alone can not be a viable revenue model.
Most of them are still trying to figure out the best way to monetize their traffic.
There are lots of ways these sites try to make money... Twitter makes some money via their iPhone app. Facebook sells gifts and also to print photos in the albums (I wonder if they ever made any money off either service).
It is not easy to come up with a good business model... In the search space there were no models until Google came up with the ad-words model. There are lots of facebook apps that make a lot of money, either by selling virtual credits, including ads, getting their users to install other applications or to buy different products... Facebook itself has yet to monotize that (they could charge app developers, but they choose not to do that).
Any business needs a good business model, and social networking sites are not different. Myspace were sold for a lot of money to news corp which thought they could figure out how to make money... It is better to have a real business model then to be sold without one, but it is a viable option if you are successful enough.