Recommended reading on Gamasutra about games for social networking sites. From the designers point-of-view:
Part of the draw of games on social networks is that their asynchronous nature is highly inclusive. Friends on completely different schedules still have a sense of playing together, and essentially groups can participate just by showing up:
The average user belongs to more than one social networking site, but devotes the majority of time to only one. As such, users have different participatory rates, logging in to one social network every day, another every once in a while, and yet another, only if an e-mail beckons the user to come back.
These different participatory rates translate into different play patterns. Some players have limited time and need a game that can be played quickly whereas others are willing to spend hours on a game. In fact, depending on the day or the social networking site selected, the same user may exhibit different play patterns.
Read on the whole feature.