Apparently FarmVille makes more money selling ringtones and credit cards and such than they do by actually charging people for things in the game. (So said Jesse Schell of Schell Games.)
He mentions around the 3 min mark that lead generation generates more revenue than direct payments (and then goes on that topic for a bit).
Mark Pincus talks about it here - http://markpincus.typepad.com/markpincus/2009/11/my-take-on-...
Peanut Labs backs it up with independent research - http://peanutlabsmedia.net/2009/11/02/survey-finds-arrington...