SOURCES: Facebook Is Working On A Huge New Mobile Ad Product

Facebook is working on a plan to allow advertisers to buy mobile ad inventory through FBX, the social network's ad exchange, two sources tell us.

A Facebook spokesperson, however, declined to comment.

FBX is a real-time bidding ad exchange in which advertisers drop tracking cookies on users' browsers as they surf the web -- shopping, for instance -- and then retarget those users with ads once they enter Facebook, perhaps to remind them to come back to the sites they were shopping on.

Currently, advertisers can only buy display ads on the desktop version of Facebook. Those ads don't appear on mobile phones, even though more users access Facebook on mobile devices than from their computers.

FBX for mobile is thus a logical extension for Facebook, allowing the company to offer ads where it users actually are.

FBX has also been one of Facebook's most successful businesses, according to observers. Some analysts believe it is already generating $1 billion a year in sales.

The obvious problem, however, is that the only ad formats available right now on mobile Facebook are Sponsored Stories and Page Post Ads. Sponsored Stories are largely text-based and driven by organic mentions from users — not the bidding, buying and selling of audiences that occurs in FBX.

So it remains unclear as to how, exactly, FBX ads would be displayed in the mobile environment. There is also the issue of mobile tracking. Cookies don't work in mobile the way they do on desktop, except in the Android environment. Apple phones don't use cookies at all — the iPhone uses its own system, called IFA.

Nonetheless, Google recently integrated Adwords, its search ad product, into AdMob, so that advertisers can make cross-platform ad buys. If Facebook can bring FBX to mobile, it might achieve something similar.