Die-hard fans will do anything for virtual cash. Zynga games often have two kinds of currency: the kind you earn by playing the game and the even more special currency you have to pay for. There are items and other in-game boosts you can only get with that special currency. As a Mafia Wars player for about a year now, I'd become a fan of Stalin for Godfather points.
Good analysis of the pros and cons of the Bing + Zynga Facebook campaign.
The real winner: Zynga, again.