You would have to think farmville would have a high churn rate to, I played it casually for about a month before I realised there was little point continuing. Even some of the most dedicated amongst my friends eventually stopped after a few months. If anything the achievements in the game seemed to keep people coming back, even though some of them were just arbitrary large numbers of things done.

I don't think it is really a fun game at all which makes me think how successful a similar game would be if someone got the fun factor right, something like a civ for facebook or something.