I see the games on Facebook as more of a convenience than a means to offer competition to an existing market.
The games are meerly a minor marketing tool to get people to log in more often. While I'm sure there are people who are addicted to some of those games, I see those games more like any other web app out there. There are free versions of everything from Snake to Bomberman out there, yet people seem to be willing to buy many of those types of titles for the mobile because of the convenience factor of having it wherever you go.
You also will encounter copyright issues, like with what happened with Scrabble. Things like this I think will hinder how far Facebook games will really go, and honeslty, I doubt it will grow much more than it already has.
I see this as more of a "bored.com" type of thing. It will have a boost in popularity, then people will find other things to occupy their time. Mobile, especially with units like the iPhone and Nokia's NGage capable devices, are becoming more like dedicated handheld consoles, which in my humble opinion will continue to exist and make billions of dollars no matter how many free options come out on the net.