Inside Social Games has a great article about covering how different companies have been using Free To Play facebook games to promote their boxed, XBLA or PC digital download titles.
"D3Publisher is promoting its upcoming summer title for Xbox Live Arcade and Nintendo DS, Puzzle Quest 2, with a Facebook match-three game, Puzzle Quest 2: Mage Trainer.
And the app is worth playing in its own right."
While primarily being a review of the Facebook Version of Puzzle Quest 2, the article has some great information about other similar titles, their monthly usage statistics and other great information.
If you're looking for a new way to promote your games, or are considering a foray into the Free To Play market then this would be a great place to start.