The move comes shortly after Twitter bought mobile ad network MoPub for a reported $350M. If Facebook does a full launch, its network will put it into competition with Google's AdMob (the market leader), Apple's iAd, and Millennial Media (MM -1.7%), which is merging with rival Jumptap.
Separately, Facebook ad sales chief Carolyn Everson says Facebook is making further efforts to simplify its mobile ad offerings (previous), and needs to do more work to improve ad measurement. The purchase of Microsoft's Atlas ad-serving platform helps on that front; so might a reportedly pending Nielsen deal.
Also: Facebook has updated its news feed ad-placement algorithm to take into account marketer feedback on whom they'd like to target, and user feedback on what ads are relevant (or not).
Facebook's mobile ad sales, for now stemming only from the company's own site and apps, rose 75% Q/Q in Q2 to $656M, and made up 41% of total revenue.