In the 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, IDC said.

Thus, it's not surprising that the U.S. government is scrutinizing Google acquisitions of online ad companies, especially of one like Admeld that is focused on display advertising, the area to which Google is extending its market dominance.

In the search advertising segment, Google had an almost 60 percent share of revenue in the first quarter, followed by Microsoft with almost 8 percent and Yahoo with 7 percent, IDC said. Search advertising is the most popular online ad format, attracting close to half of all spending, while display is the second most popular, but the fastest growing, according to IDC.

Simultaneously, the U.S. Federal Trade Commission is also probing Google's dominant position in search advertising, Bloomberg reported.