With the comments from some analysts today that social gaming is flat or unsustainable, let me mention something that may be clear to people on the inside but might not be visible to the outside:
Social games, and mobile / iPhone games, are designed to exploit platform bugs.
What does this mean?
All of these bugs - and many others - are in the process of being resolved by the platform owners. When these bugs are corrected, games will have to compete in an entirely different ecosystem than the one they were familiar with before. Their process of natural selection will suddenly have to adjust to different fitness criteria. Some games may persist, other games may not.
What many companies do, however, is constantly push the edge of the bugs and attempt to refine their viral appeal. This kind of A/B testing and focus-grouping generates the highest possible effectiveness out of every exploit, but separately publishers still have to generate the kind of lasting appeal that builds a franchise people want to play.