A lot of teachers claim that games are a big part of the distraction. It might be difficult to make a pupil stop gaming during class, if there is not a pause button on a specific game he is playing. Facebook can easily be turned off, but a pupil might cause a scene when a teacher asks to stop playing a game that does not have a pause function.
The answer is more or less the same as in the previous part about distraction in general. You have to be transparent on how you want your pupils to use the iPad. And if they cannot obey the rules or cause a scene, you should consider punishing them in a suitable way.