For one thing, the WSJ notes, comScore’s numbers leave out ads that Yahoo and Microsoft distributed across syndicated sites, which contributes a great deal of those sites’ display revenues. Facebook reportedly brought in $500 million in revenue last year and is slated to double that. Meanwhile, Yahoo had $6.5 billion in online ad revenue last year, while Microsoft’s was about $3 billion.

Although Facebook is still way behind both Yahoo and Microsoft in terms of ad revenues, its growth shows no signs of slowing. And it also boasts rising “engagement” figures in terms of users’ time spent on the site. All of which has helped it attract some first-time major advertisers, including Ford, Verizon and Toyota.

The comScore data matches previous measurements from Nielsen, another measurement firm, which said that Facebook’s share of the U.S. display market was 20 percent by the end of April 2010—a significant leap from the 2 percent the same time last year. And like comScore, Nielsen doesn’t include the impressions served by Yahoo through syndication. Neverthless, it shows Yahoo in the lead when it comes to ad serving, with 34 percent of all display ads, which represents a slight decline in last year’s 35 percent share.

In total, comScore figures show that display ad impressions rose 16.5 percent to 1.1 trillion in Q1, which is just another sign that after being written off for the past two years, display is making a comeback, even if the lion’s share is starting to shift from the portal sites to Facebook.