Ego, pride... ;-) I am not actually a facebook user myself, I don't even like it very much.

Another concern is monetarization - even if I have more users on facebook, would I be able to earn similar amounts of money per user as with my own website?

I also wanted to get out of the dependency on big corporations. So far most things I did were dependent on somebody controlling the market. With mobile games, there was the dependency on the mobile phone carriers and established games vendors, for example. Overall I was unhappy with that situation.

I haven't made the decision yet, because I only thought about the facebook aspect today.

Zuckerburg has been pretty big on promoting favor towards facebook app's, so it's probably not as bad as cell phone development (I had a friend in the mobile games market, they just aren't as friendly as they should be).

Even if you don't get the same monetization, you're going to be able to get a lot more users by building into the biggest existing social network than having them switch/use a new one.

Even if you made 1/10th the amount of money, if you can get 10 times the users then you're still good.