In the facebook paper, this was used for gauging popularity, the studying of power-law over the apps, etc.
Subscribed users in PK: hard to determine for sure since not all the databases have been recording data since the game's release
We can find active users by first defining what an active user is, and then using the 'logins' and 'sessions' tables to identify those users.
It may be useful to determine the percentage of regularly active users within each platform (iPhone vs. iTouch vs. Android) or percentage within each speed bracket (3G wireless, etc.). However, we do not have specific network speeds for each unique user.
It is possible to make a connection to power-law like the paper. This will be something we may look into as research continues.