Back in the days, there were some small ad networks that used to run some very manipulative ads, or even popup ads on facebook applications. Since then facebook has imposed restrictions on what kind of ads are allowed. In the latest iteration, they imposed a huge list of draconian terms http://developers.facebook.com/ad_provider_terms/ . The terms seem to be designed so that no large ad provider (who is serious about its customers) can agree. Seemingly, developers will not be able to make their own advertising deals unless facebook agrees to approve it.
Google adsense was so far one of the best options for developers: they provide the safest kind of ads and their payout is much higher than the approved advertisers. Repeated attempts to get google or facebook to comment on the matter are left unanswered. Until now, developers were in limbo, as facebook vaguely suggested they are still in talks with google. Now it seems the decision has been finalized. This is basically an "FU" from facebook to its developers, depriving them of their best monetization options without providing an alternative (i.e. facebook ads revenue share).