Information about your application is available through the Admin.getMetrics API call. I believe that it provides only the number of active users for a certain period of time.
The actual total number of users is difficult to track due to people who uninstall the application or can be installed but blocked. I do not think that it is available as a specific property anywhere. The information pages about Facebook applications never show the total number of users, only monthly active users, which Admin.getMetrics() can tell you about.
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