From Techcrunch and referring to the US:
During 2008, comScore estimates that 4.5 trillion display ads were served in the U.S. alone. That comes out to more than 2,000 Internet ads per month per person. And, believe it or not, the number of ads served up actually declined a little during the year as publishers tried to push up CPMs (the amount they can charge per thousand ad impressions) by reducing inventory.
