This is unfortunately but has been happening to many players, especially those with a lot of friends.
The game require a lot of memory to run. If you are on the original iPad, you have a much greater chance of crashes. iPad2 is less prone due to more memory. The new iPad is better in this regard.
If your game crashes frequently, close out of the game. Close out ALL background apps (double click home, hold one of the app icons until X appears, click X on all apps). Once you shut down all background apps, restart your iPad.
Once it is done rebooting, start up MM. Performance should be better now, and perhaps it won't crash as often.