Edit: The thought occurred that Facebook may be blocked in China, and it appears that it is (or at least was for awhile? My hits were all for one instance of them blocking it, but I couldn't find anything about whether it stayed blocked). So that could be part of the reason for doing this.
Several sites on the internet number below or around Alexa 1,000 catering to these players - ArmorGames.com, AddictingGames.com (MTV), Kongregate.com, MiniClip.com etc.
For as many users as Facebook has it's still only 1/4 of the internet.
But that's true that it's only 1/4 of the Internet.
One game of mine that did about 1.2 million plays on portals peaked at I think 400 users on the Facebook version, which was promoted within the portal version.
Recently King.com launched a big facebook app called Funflow, which they linked to from over 100 games in ditribution and several large, many-millions-of-plays new releases. They've probably had close to 100 million plays across their games this year, yet their fb app hasn't quite hit 1 million users.