• by Joe Mandese  @mp_joemandese, September 20, 2012

Google is poised to become No. 1 -- again. Long dominant as the online search advertising giant, Google is on track to become the dominant player in the online display advertising business too, according to estimates released late Wednesday by online stats masters eMarketer. Actually, according to the eMarketer report, Google will achieve digital marketplace triple play -- becoming Madison Avenue’s top supplier for three digital marketing channels: search, display and mobile.

“Google now holds more share than any other company in each of the U.S. search, display and mobile advertising markets,” eMarketer reports -- noting that the display milestone is based on Google overtaking previous No. 1 supplier Facebook, which only overtook long-standing display ad champ Yahoo last year.

The leap-frogging is based on a projection that Google will take in $2.31 billion in U.S. display ad revenues this year, a 38.5% increase from 2011, and giving it a 15.4% share of the total display marketplace.

Facebook, by comparison, will earn $2.16 billion in U.S. display ad revenues this year.  And while that also is an impressive rate of growth -- jumping 24.4% over 2011 -- it’s not enough of a share gain to offset Google’s momentum. Facebook, however, picked up 0.3 points of market share to further distance itself from third-place Yahoo with a 14.4% share of the U.S. display ad marketplace.

Yahoo will drop to 9.3% in 2012, down from 11.0% last year, and a dominant 14.0% of display ad budgets in 2010.

Microsoft retains its fourth-place position, dropping 0.4 points to a 4.5% share, while AOL remains No. 5, declining 0.7 points to 3.6% of the online display ad marketplace.

Google’s and Facebook’s ascendency comes amid a rapidly expanding online display ad marketplace, thanks in part to Facebook’s push. eMarketer estimates the total U.S. online display marketplace will be $14.98 billion this year, up 21.5% from $12.33 billion last year.

Looking forward, eMarketer expects Google to continue to build share of display ad budgets, expanding to a 21.2% of display in two years, and increasing its lead over projected No. 2 Facebook’s estimated 15.5% share in 2014 by a wide margin.

eMarketer said its estimates were “driven by the expansion of both Google and Facebook as advertising platforms; the continued health of banner spending due to expansion of inventory, aided by mobile growth; and increased spending growth on digital video advertising, especially YouTube.”

Despite that growth, eMarketer has actually reduced its projections downward slightly from its previous display forecast due to a “mix of lower prices for display advertising on ad networks combined with the reluctance of some major brands to make extra-large investments in digital display advertising.”

“There are several factors underpinning Google's ascent to market leader,” eMarketer noted, adding, “including the continued strength of its ad network, video advertising on YouTube and mobile display advertising on AdMob.”