Farmville's wildly successful by any metric. It's an okay game, but I think thta Zynga has done a really good job with their latest, Frontierville. It's actually a fun little casual game.
The spammyness of these games can be controlled by setting up a friend's list specifically to send in-game messages out to, but that is a feature I really wish the games themselves would incorporate. I don't mind spamming my friends who also play these games, because they want to click on the links to get the goodies...but I don't like hammering my non-playing friend's walls with hundreds of game related posts a day. That's way beyond viral marketting and deep into harassment territory.
I wouldn't call this relationship parastic as much as symbiotic. The main reason I frequent facebook as much as I do now is because of Zynga's games.