Interesting analysis and speculation: F8ce the music. The articles includes this article, which shows that a number of top Facebook apps have now fallen from their peak:
It strikes me that a lot of these apps have now gone through most of the "carrying capacity" of the Facebook audience. Everyone’s seen them now, and the novelty has worn off a bit.
Definitely worth thinking about more – how do you create apps that are both viral and are good at driving engaging experiences that last a long time?