Developers can incorporate user-tracking code provided by companies like Pinch Media or Medialytics in their applications.
Pinch Media's site says they can "track the number of unique, active users, the length of time your application is being used, and, if enabled, the geographic location of your users."
(I for one wish there was a way I could block such tracking.)
But undoubtedly these companies have a lot of information on what apps are truly "popular" in terms of usage and such. I wonder if these tracking companies will ever publicly release the data on what apps get used a lot and such?