If you were using Google Adsense to generate revenue within your Facebook application, you no longer can according to the social network's official policy, Facebook developers are now not allowed to monetize their apps with Google AdWords. The problem seems to be about Google's decision to not accept Facebook's advertiser agreement, which includes a variety of rules about how advertisers can operate on Facebook's platform.
Advertisers had until Feb'28th to accept Facebook's terms. Those who did are listed on Facebook's developer site, but Google is missing among the dozens of advertisers.
Here's Facebook's statement:
This is the current list of ad providers that have signed our terms that govern ad quality and data use, although more are being added every day. We currently have more than sixty of the largest and smallest providers on the list. This is a public and open agreement and new providers can sign at any time.
It's unclear if any developers that continue to use AdWords will be banned. The blog post points to a couple forum threads (here, and here) where developers are discussing alternatives that include abandoning Facebook altogether.