Could this just be a gap in between new games? Zynga games do seem to be sticky and addictive but there is going to be a lot of attrition the longer a game is around.
Yes, I imagine that could definitely have something to do with it. I also think part of it is that others (Sims Social) have shown they can build sticky and addictive games as well and Zynga is no longer the sole owner of the space.
Most of Zynga's games differ only very superficially though. It's weird that they can't launch games fast enough to stave off attrition.