Here's an interesting web site / company.
Adonomics an open analytics and advertising platform for Facebook
(Note, they state that their site is in no way affiliated with Facebook)
It's well worth going to explore this site which contains a weath of statistics and information about applications in Facebook.
Their leaderboard shows the total (world wide) numbers of users of particular facebook apps. The board shows the total number of installs per application, the number of active users of an application and the percentage of active users.
Their charts page shows a top 25 list of facebook application companies, clicking on the company name provides a list of applications and a to show daily active users graphed over time.
A superficial examination of random graphs reveals that many of these apps have declining user numbers.
Go see for yourself.
Adonomics offer their services to application developers, and provide a Marketplace for application sellers.
Those of us who are curious about Facebook, its users and its applications can find much to explore here.