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.