Yeah, but that's wrong assumption. This is no one-way street.
I'm counter-example myself. I actually reverted from being hardcore game player to playing mostly simple web games.
And I sure know others which went this way - from AAA titles to Zynga.
Here I said it. Most "real" gamers would be probably too ashamed to admit it :).
And about growth - I'm not sure what is the cause and what is the effect. In non US regions games themselves could actually be the driving mechanism for Facebook growth:
http://news.ycombinator.com/item?id=989607