I found yesterday from a link at RockPaperShotgun two essays which were very interesting to me. Their main field was to look into social gaming, and how it entraps people, and furthermore how much of the industry is entirely based on that idea.
While long, they are certainly worth a read.
The first one can be found here, and speaks of the general trend in social gaming.
The second one at here is a more in-depth review on The Sims Social.
I will finish by just adding that I have never felt the wish to play any additional games on Facebook. Much as said there, Facebook itself feels like a game some of the time. I also feel that they would just spend my time without providing the (wonderful) immersion some proper games do (or rather, that proper games do more easily).