Most actual Facebook users knew the deal, but today, Facebook released "activity" numbers for its apps and the results aren't pretty.
Instead of lots of applications showing millions of "users", only one single application has over a million daily users, and that's Top Friends. Most of the applications had around 10% of their userbase that actually interacts with the application in any given day. Knowing that most Facebook users average logging into their Facebook accounts everyday, this isn't really that much usage.
For example, the ultraviral "Zombies" app only has about 3% of its installed base using it on any given day, making its actual usage numbers around 90,000 people, not 3 million.
On the other hand, the "Are you interested?" application has about the same number of active users, but this represents a full third of their userbase!
This is a great way to show statistics on usage and also puts this Facebook phenomenon in a little bit of perspective. Nice move!
Now, all these developers can get to work creating apps that more than 10% of the people that have them want to use in any given day.