• by Mark Walsh  @markfwal, April 29, 2011

Facebook accounted for more than one-quarter (25.8%) of all U.S. display ad impressions in the fourth quarter of 2010 -- up from 23.1% in the prior quarter, per comScore. The social network's gain came at the expense of Yahoo, whose display ad share fell to 9.7% from 11%, and Fox Interactive Media, which dropped to 2.4% from 3.8%, in the third quarter.

Among other top Web publishers, Microsoft's share slipped to 4.5% from 5%, and Google, from 2.7% to 2.4%. AOL saw a slight uptick from 2.4% to 2.5% in the fourth quarter. More than 1.3 trillion ad impressions were served in the quarter, about the same as the prior three months. Of that total, Facebook was responsible for 338.6 billion.

The new figures reflect Facebook's growing dominance of display advertising based on its more than 500-million-strong audience worldwide and its advantage over the major Web portals in user engagement. According to Nielsen, the average time spent per person among Facebook's 135.7 million unique U.S. viewers in March was six hours, 35 minutes, compared to 2:26 for AOL, 2:16 for Yahoo, 1:26 for MSN/Windows Live/Bing, and 1:21 for Google.

Facebook surpassed Yahoo last year in ads served to become the country's top display ad publisher. Last month, eMarketer issued a report forecasting that this year Facebook would also overtake Yahoo in display ad revenues, at $2.2 billion, or 21.6% of all U.S. display ad dollars. The research firm expects Facebook's overall ad sales to reach $4 billion in 2011.

The social network and other sites benefited from a strong rebound last year in display ad spending, which increased 24% to $9.9 billion, according to the Interactive Advertising Bureau. In terms of ads served in the fourth quarter, Facebook saw a particular spike in December, when it delivered 128.1 billion ads, or 28.1% of total impressions. By comparison, Yahoo accounted for 9.3%, and Microsoft, 4.7%.

During the quarter, Fox had the steepest slide, droping from a 3.3% share of ad impressions in October to 1.7% in December. The decline likely reflects in part dwindling traffic and advertising on MySpace, which owner News Corp. is looking to sell in light of Facebook vanquishing its one-time social media rival.

Despite what eMarketer predicts will be an 81% surge in its display ad revenue this year, Facebook has recently taken steps to boost spending by large brand advertisers on the site. To help lead that effort, the company last month hired Mark D'Arcy, former president of Time Warner's Global Media Group, as director of global creative solutions. It also launched Facebook Studio to showcase successful creative campaigns and the social network's ad tools.