Important - The figures only show active users of the app on Facebook, not usage of the mobile app
Source: Data from AppStats, retrieved 14th January 2013
Note - You can see a few things in the data:
1 - The drop in daily active users when Instagram announced the changes to their terms & conditions (which they then backed down on). It has never risen to the peak it hit the day before this.
2 - The levelling off in the number of active monthlies at this point
3 - While the active monthlies are still growing it could be that these people are less likely to be hardcore users, and that a number (1m? 2m?) of the hard-core, daily users left in late December
Note 1 - Facebook have disputed the figures but not volunteered their own numbers