Facebook will then sing it big together with Bing, which might work towards Bing's advantage. I can see in a future Bing and Facebook eating more and more into Google's advertising dollar. Which can only be good as in competition is good.
The funny thing is that Microsoft's adCenter isn't on the list of approved providers, which is a little surprising given that Microsoft and Facebook already seem to have a decent relationship.
Facebook doesn't want to be a Microsoft partner, they want to be AOL, circa 1994. (ie, a walled garden)