I love Facebook games, but there are a few popular ones I just don't understand why people play.
Top Restaurant -- While I can see the appeal of a game which amounts to virtual whack-a-mole, where you are constantly putting out a series of 'fires' in order to stay ahead, this (and most other restaurant applications) is a little too much like running an actual restaurant. Anyone who has ever worked food service will rightfully shy away from such a game. Some of them, such as Top Restaurant, add to the horror by encouraging one restaurant owner to steal tips off the tables of rival restaurants.
Aquarium Games -- Having a virtual aquarium is nice as a screensaver, but when it becomes something like a fast-paced virtual pet? Not so fun. Sure, you get gold for cleaning your aquarium, but the downside is these aquariums can get dirty at a phenominal rate. Who in real life wants to clean a fishtank four times a day? And in most of the games, the damn fish still die.
YoVille -- I am utterly baffled why this game is so popular. The Sims rocked for several reasons, a sort of virtual voyeurism coupled with their amusing behavior being two. But all the characters in YoVille look like prepubescent gangsta wannabes, while the whole point of the game is to get the best wannabe clothes, with the best home interior, by working at a bakery where you have to clean ovens. You get points (and are encouraged) to constantly message your YoVille friends, and since no one has anything meaningful to say (in fact, they often don't know each other outside of facebook games), things devolve into postcards saying, "yo, yo, YOOOOOO!" and "hey, whaddup?" back and forth to infinity. Why does anyone want to come home from work in order to log on and do virtual work? Or hang out with folks who want to be wannabes?