Addiction via immersion and frequent incremental rewards.
One: you immerse the user in the game environment so that all of their attention is focused on the game world.
Two: using that focus, you provide a system wherein there is the constant availability of incremental rewards. In games like WoW, every single minute spent goes toward achieving a specific incremental reward. You kill a monster: you get a reward (loot + experience), you go to the nearest town: you get new quests. The key is to have a near-term action (< 5 minutes) available to the user at ALL TIMES where they can achieve some kind of small reward. Larger incremental rewards (levels, skills, raids) are available to keep attention once the smaller rewards become rote.
Look at anything that's ever been called addictive- not just video games. I bet that they share these two traits, and their addictiveness is directly proportional to how well they accomplish these goals.
Can these be applied to a facebook app? Maybe. Would you look sleazy trying to do it with a facebook app? Definitely.