Apps on Facebook.com - Facebook Developers: "Bookmarks for quick re-engagement
Once a user starts using your app, we create a bookmark to enable users to easily navigate back to your app from within Facebook. These Bookmarks are available on the Facebook homepage as well as on the Canvas Page on the top right corner.
These Bookmarks are ordered based on how often and recently the user used your app. The red counters on the bookmarks notify users of outstanding Requests 2.0 related requests for your apps and encourage them to respond."
'via Blog this'