Last month CNN Money put up an article titled The Facebook Economy. The Article is very worth reading. It talks about people and companies writing applications for Facebook and the business model involved.
It is not a surprise that most of them base their model on advertisement, but what is a surprise, at least for me, it is the number of Facebook users that a single application can have. There are some numbers in the article in the range of millions.
Third party applications on Facebook are possible because the social website had opened their platform making available an API that anyone can integrate with and also monetize it, which had attracted many developers.