A very interesting article on CNN money on the facebook economy.

How to turn social networking sites in money making machines is what many companies are trying to do. At the moment only a few web sites are able to monetize the phenomenon using essentially advertising banners. The article describes with a rich set of examples the way different companies, applications and services have generated revenues.
There is nothing obscure but just simple rules to take into account if you want to enter the arena of social networking.