I don't think there is a lot of crossover, I've published a few casual games that promoted things like Twitter and Facebook apps, and the conversion rate from portals to twitter and fb has been horribly low every time.
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.
You have to understand that there is a difference between real Facebook users and older users who just made accounts so they could feel that they were participating in social networking. Real Facebook users have tons of personal information on their accounts. They are more reluctant to use Connect to login to other websites for the same reason that people don't use their real names when making user names. Older Facebook users don't have the type of network to actual get much use out of Facebook. They actually think Facebook is a place to meet people, a notion a real user would consider completely ridiculous. Older users don't view Facebook as an online portrait of their offline lives and are more likely to use Connect.
But that's true that it's only 1/4 of the Internet.