You should separate the notion of the interaction modalities (mouse, touch screen, voice, etc), from the ways they can be used in various different types of sof

You should separate the notion of the interaction modalities (mouse, touch screen, voice, etc), from the ways they can be used in various different types of software. New modalities such as touch screens will require designers to rethink how the user interacts with their software, but it doesn't change the goals of the user. From a user's perspective, games and productivity software are fundamentally different. The point of games is to have fun. The point of productivity apps is to get things done. I saw the lead designer of Halo give a talk at school several years ago, and he used this example -- 'if we designed games to be like productivity apps, then the ideal game would consist of a big red button which said "Press me to complete game!" ' Social networking applications lie somewhere in the middle of the spectrum. You're not necessarily there to 'get things done'. You're there to linger as in a game - yet you do have more concrete goals and knowledge of your path/environment than you would if you were in a game.