Facebook platform developers could see $500M in revenue this year » VentureBeat
http://venturebeat.com/2009/05/08/facebook-platform-developers-could-see-500m-in-revenue-this-year/

A growing number of game makers on Facebook are making money from virtual goods — from poker chips to virtual clothes that users can buy or earn while playing gaming applications with their friends on Facebook. The combined ecosystem of these game developers and other companies supplying services to them could generate half a billion dollars in revenue in 2009.