7 billion ad impressions are served each day on the Facebook ad exchange

"Appnexus, the online ad marketplace intermediary, hosted an annual summit in New York yesterday and disclosed some interesting metrics about the size of its business as it relates to Facebook.
The company is one of let slip that Facebook Exchange -- the real-time bidding ad exchange in which advertisers are allowed to match their cookies to a Facebook cookie in order to retarget shoppers inside Facebook -- now serves a staggering 7 billion ad impressions daily. According to AdExchanger:
AppNexus is quickly ramping up on the Facebook Exchange, where it is one of approximately 15 real-time bidding partners. It now has exposure to 7 billion daily impressions on FBX, up from zero in July — a large and important new inventory source."

Source:  Business Insider, 8th November 2012