When I started playing Mafia Wars on facebook, the game was very much centered around input in the top area of the screen. Over time, Zynga have added their own bars and gadgets at the top of the screen, pushing the game content down. In more recent additions to Mafia Wars, you often find yourself spending large amounts of time scrolling down a long way, so much so that the top part of the page is never seen.
And this means that the ads that facebook throws at you are also not seen. Whilst this is a good thing for me (I'm not bothered by ads whilst I play), it also means that those paying for those ads are also not getting what they think they're getting. Why? Although I might be present on a given page for many minutes, as the ads are never seen even though they may be present for many minutes,. Thus a page impression from Mafia Wars results in me seeing the ad for a vastly shorter period of time than I would if it were facebook itself.
With other games such as FarmVille, the action is very much centered on the main playing screen, such that you don't have to scroll away from the ads.
If facebook and Zynga are in bed together to keep customers around, then it would seem that it's the third party in this menage a trois - the advertisers - that are getting screwed by both of their bed mates when it comes to Mafia Wars.