I built a few large scale viral games at zynga, crowdstar, etc. These games eventually exhibited the same strange behavior. They would stop growing - then sta

I built a few large scale viral games at zynga, crowdstar, etc. These games eventually exhibited the same strange behavior. They would stop growing - then start a quicker decline into nothingness. We tried every thing: features, advertising spend(s), focus groups, etc. Once all your users' friends and contacts have been exposed to your game - growth becomes impossible. They either didn't want to try it or tried it (immune / infected). Once you have peaked, people with the least interest begin to leave. The scary issue is that people with moderate interest are effected by this group of early leavers. Then moderately interested users start tapering off. This begins a very quick death circle.