I don't "play" Farmville, but I have to wonder if it will see the same kind of success when (if) people need to log in to a separate website to be a part of it. I imagine that part of the allure is integration - while you do whatever else you do on facebook, you can do the Farmville thing, easily send messages to your friends/post to your wall about how you need them to water your goats, baconify your pigs, etc., and send obnoxious notices every time a new blade of grass starts growing.
Will the hype sustain if people have to navigate elsewhere for their fix? Obviously achievements and updates are present in a lot of games, but I'm curious to see if dislodging it from Facebook will cause a meaningful drop in the number of users.