Facebook ad sales chief Carolyn Everson. Bernstein Research analyst Carlos Kirjner calculates that Facebook's ad exchange — which shows ads to Facebook users triggered by cookies that have tracked them from other web sites — is on course to generate $1 billion a year in revenues.

He also estimates that the exchange, named FBX, could take up to 50% of the entire market for ads generated by automated real-time bidding.

That's a staggeringly massive market — and Facebook only created it in June, just five months ago.

Previously, Facebook COO Sheryl Sandberg told investors she believes the global RTB market was worth a total of $2 billion. If Kirjner's numbers are correct, then Facebook could claim half that entire business.

Here's a paragraph from his extremely detailed note to investors:

We are not aware of any reliable sizing of the current amount of display advertising spend placed through retargeting, but we believe it is in the rough order of $1 billion per year globally (gross), as it will correspond to only a portion, probably in the order of 30% to 50% of total real-time-bidding (RTB), or "programmatic" spend. The rest of the RTB transacted impressions is sold without cookies (which are required for retargeting) or is targeted based on publisher and third party data, and not on a users Web history. It follows that if a large portion of Facebook's roughly $4B (and growing) in annual advertising revenues were to be placed through retargeting, it would represent to a large acceleration to the RTB market.

Disclosure: The author owns Facebook stock.