If I wasn't married, and I didn't mind exploiting human beings to make money, I could make a real addictive game like Farmville.
You put in random "awesome item" drops, sell in-game items on the game's website, make a player's hard work deteriorate if you don't log in regularly, etc.
I've read many articles which discuss how to make an addictive game -- I posted them here, as a matter of fact.
Matthew