U.S. Internet users saw an eye-popping 4.3 trillion display ads last year, with one company standing apart from the crowd as the runaway leader in graphic advertising.

Yahoo (NASDAQ: YHOO) held a commanding lead over the next-closest display advertising network, serving up 521.2 billion ads on its sites, according to a new report from online metrics firm comScore (NASDAQ: SCOR).

Fox Interactive Media, the division of News Corp. that includes MySpace, checked in at No. 2 with 367.6 billion ads, followed by Facebook, which served up 329.6 billion placements.

In total, comScore reported a 21 percent increase in the number of display ads served on U.S. sites in 2009. In measuring the category, comScore counts both static and rich-media graphics, but does not include video ads.

For Yahoo, the display segment figures to play a central role in the company's ongoing turnaround mission as it looks to build out its content verticals and boost sales of premium inventory for branded messages.

On comScore's list, Microsoft (NASDAQ: MSFT) was the fourth-largest advertising hub, serving 218.1 billion display ads on its network of sites, followed by AOL, with 192.2 placements.

Google (NASDAQ: GOOG) checked in at a distant sixth on the list, with 69.9 billion display ads.

Among advertisers, three of the 10 biggest spenders were telecom providers. AT&T (NYSE: T) shelled out for 84.3 billion impressions, followed by Verizon Communications (NYSE: VZ), which bought up 56.8 billion spots. Sprint Nextel (NYSE: S) was tenth on that list with 26.2 billion ad impressions.

The Social Networking Horse Race

Looking at other segments of the digital world, comScore described 2009 as a "landmark year" for social networking. In December, comScore found that nearly 80 percent of Internet users visited a social networking site, and that social activities now account for 11 percent of people's time spent online.

The leading storylines in 2009 were Facebook and Twitter, which both enjoyed triple-digit percentage growth in their user bases, with Facebook surging past MySpace by measure of U.S. visitors in May, after having overtaken its rival globally the previous August.

For Twitter, the rapid growth came in the first half of the year, while in the later months the microblogging site saw its traffic hit a plateau. Still, Twitter began 2009 with just 2 million users, ending the year with nearly 20 million.

For MySpace, 2009 was a rebuilding year. The one-time leader in the space shuffled its executive leadership, installing Facebook veteran Owen Van Natta as CEO after the departure of co-founder Chris DeWolfe.

MySpace executives have signaled that they are no longer trying to beat Facebook at its own game. Instead, they have focused on building out the site's entertainment platform, a strategy that comScore said is off to a promising start.

"MySpace has experienced some softening in its audience," the company said in its report. "However, a new strategic focus on entertainment content is exhibiting signs of success with MySpace Music having grown 92 percent in the past year."

But comScore's litany of usage statistics on social networking sites tells the story of Facebook's increasing prominence in people's online lives. In categories like total visits, minutes spent on the site and average daily visitors, Facebook enjoyed triple-digit percentage growth. The only metric in which Facebook saw a decline was in average minutes per visit, which comScore said was the likely result of the "increasing frequency with which people are visiting the site."

Kenneth Corbin is an associate editor at InternetNews.com, the news service of Internet.com, the network for technology professionals.

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