In this year's first quarter, Google's share of the U.S. display ad market rose to 14.7 percent from 13.3 percent in 2010's fourth quarter, while Yahoo's share shrunk to 12.3 percent from 13.6 percent.
The news has to be encouraging for Google, which for many years was a minor player in display advertising. However, it has made big investments to improve its position, including spending billions to acquire DoubleClick and YouTube.
For Yahoo, this development is troubling, since display advertising is its core business, offsetting its underpeforming search ad business.
Google dominates the search ad business with a share of almost 60 percent in the first quarter, followed by Microsoft with almost 8 percent and Yahoo with 7 percent. A controversial search pact between Yahoo and Microsoft to counter Google's dominance has gotten off to a slow start, with Yahoo complaining that its revenue-per-search (RPS) isn't meeting expectations in markets where it is already using Microsoft's AdCenter platform.
A bit of good news for Yahoo and other traditional display ad providers like AOL and Microsoft from the IDC report is that Google's growth in display ads hasn't come at the expense of their own display businesses, which are powered by mostly by large, brand advertisers. Google is mainly tapping its core small and medium-size search advertisers to also spend on the Google Display Network (GDN), according to IDC.
"But that may change, and when that happens, the old stalwarts' lumbering sales organizations will have to compete with GDN's ultra-effective automated sales interface," IDC said in a statement.
Although it has unseated Yahoo from the top spot, Google needs to keep an eye on Facebook, which IDC expects will make a strong push in display advertising in the coming years.
The IDC study also found that online ad spending grew 14.3 percent worldwide in the first quarter to $18.2 billion, while U.S. spending grew 14.2 percent to $8.1 billion.
In a separate study also released on Thursday, the Interactive Advertising Bureau and PricewaterhouseCoopers pegged U.S. online ad spending growth at 23 percent to $7.3 billion in the quarter.
Search advertising continues to be the most popular ad format, attracting 48.7 percent of the quarter's spending, while display advertising was second with 33.3 percent. However, over the past two years, display has been growing faster than search, according to IDC.