Facebook accounts for 28% of online ad impressions in the US

"Ahead of a rumoured IPO as early as tomorrow, Facebook’s position in the ad space has been boosted by comScore data that shows its share of US ad impressions grew to 27.9% in 2011.
This is up from 21% in 2010 and means Facebook has been ranked number one in comScore’s display ad impressions category for the past three years.
Despite its well-publicised woes Yahoo is second on the list with 11% of the market, while Microsoft, Google and AOL trail with less than 5% each.
Unlike the other companies on the list Facebook is almost totally reliant on banner ads to bring in revenue."

Source:  Data from comScore, reported by Econsultancy, 31st January 2012