My game is one week old and 38 users have downloaded the app so far. But if you download and play the game the number of players in the Game Center leader board is not nearly as much. The Lite version has enjoyed almost 200 downloads in 2 days but there are only 5 players on the leader board. This mystery leads me to the following hypothesis:
1. The leader board is per country.
If the LB is per country naturally a player will only see scores of other players in his or her country.
2. Not all who downloaded actually played the game.
I would like to assume that sometimes people download the app but never open it.
3. Not all who downloaded have Game Center enabled devices.
Game Center requires iOS 4.1 and will only install on iPod Touch 2nd Gen and iPhone 3GS and later.
Conclusion: The Game Center Leader Board is not indicative of the number of users of an app.