Google Inc. is set to become the biggest earner in U.S. display ads this year, taking the No. 1 rank away from Facebook Inc. and cementing its dominant presence in online advertising.

Google's rising clout in display ads — the boxes with images and video found on websites — has come on the back of growth in video advertising on its YouTube subsidiary, as well as mobile advertising through Admob, a company acquired by Google in 2009.

This year, Google will claim 15.4 percent of display ad dollars, or $2.31 billion, compared to Facebook's 14.4 percent, or $2.16 billion, according to projections by eMarketer, a digital advertising research firm.

But it added that publishers across the board have been bruised by weaker-than-expected display advertising demand this year as large brands withheld splurging on costly digital campaigns.

Top display ad earners Google and Facebook were followed by Yahoo Inc., Microsoft Corp. and AOL Inc., which all have advertising networks across their digital content properties.

Facebook said this week that it would begin developing a mobile ad network that would allow advertisers to target ads using Facebook data across the web and not just Facebook's website.