The Art of Gifting, Challenges, and Notifications
My friend Hiten Shah's company, KISSmetrics has recently done a series on how popular Facebook apps use notifications effectively. It's an excellent introduction and there's some good insights.
The real value of the posts come from KISSmetrics detailed diagrams of the notification flows from popular games, mostly from Playfish. Check it out.
The Art of Gifting
The Art of Challenges
The Art of Notifications