Google has not agreed to comply with Facebook's Platform Terms of service, and therefore developers trying to monetize with its ad networks are required to find other means from a list of networks that have agreed.
According to Inside Facebook, Facebook's deadline for developers to use only ad providers from its list was the end of February, but now they're starting to enforce it, asking developers to switch.
Current networks on the whitelist include: ad:C media, Ad4Game, Adap.tv, Adconion Media Group, AdJug, Adknowledge - (Cubics and Super Rewards), Admeld, Adperio, ADTELLIGENCEm Advertising.com, Alfy Video Network, Appatyze, Applifier, APP Prizes, appssavvy, BannerConnect, Bizmey, Blue Noodle, BrightRoll, Casale Media, ClickForce, CPX Interactive, Criteo, DoubleDing, DSNR Media Group, DSNR Media Innovations (DMI), EpicSocial, Fox Networks, Harren, Media, Httpool, igapi by Social Game Universe, IndieClick, Intergi, Jun Group, Lifestreet Media, Live Gamer, MakeMeReach, Malepar, MATOMY, Mediastay, MediaWhiz, Nabbr, One iota, Open Inventory media, Paymentwall, Peanut Labs, PubMatic, RadiumOne, RockYou, Rubicon Project, Sharethrough, smartclip, Smowtion, Social Games Ad Network - SGA, SocialVideoBytes, Sometrics, Specific Media, SponsorPay, SpotXchange, SupersonicAds, SVnetwork, Tapjoy, Techcrea Solutions - FirstHeberg, TokenAds, TrialPay, Underdog Media, Unruly Media, ValueClick, WildTangent, XTEND, and Ybrant Digital.
Facebook recently told AllFacebook, "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."
We haven't seen any comment from Google on the subject, but it's becoming increasingly clear that these two companies are direct competitors in more ways than one, though neither company likes to address this fact. Should Facebook choose to expand its own advertising offerings, things should really start to heat up.
On a semi-related note, Google has announced a new update to AdSense to improve speed. They say AdSense doesn't impact your rankings, but speed is a signal, so this could play some minor, indirect role.