I insta delete games that require Facebook login... But I suspect your typical Facebook whale isn't the Toucharcade demographic.
We have a couple of iOS games that started out as Flash games, which we put on Facebook as well as on various portals. The Facebook versions had a social highscore system (you could challenge friends, see how you ranked, etc.), but not much else in terms of viral mechanics. These games did well on the web, but they didn't perform well at all on Facebook. Maybe the situation is different if your game is inherently social, or if you have Dollars to throw at ads so you can get the ball rolling, but if all you're going to do is take an arbitrary casual game and put it on Facebook, I think you're probably wasting your time.
Facebook Facebook games, or games ported for Facebook seem to have a lower rate of user acquisition then the app stores. On the other hand, if you have a very strong Facebook presence, and the ability to get the word out about the game to a large network of Facebook friends, then it could pick up quickly. Just depends if you are willing to put the effort into the game to make it a hit on Facebook.