Nielsen//NetRatings today released its February U.S. Internet search market share data (PDF) and Google continues to dominate, increasing its lead dramatically faster than its nearest competitors, Yahoo! and Microsoft.

Results show 55.8 percent of the estimated 3.6 billion U.S. searches were done with Google in February. That's up from 53.7 percent in January. Yahoo! claimed 20.7 percent last month, down from 22.7 in January. Microsoft had 9.6 percent, up from 8.9 percent.

The growth rates of the big three further illustrate the challenge Microsoft faces in catching its two major rivals. Google's share grew 40.3 percent year over year in February. Yahoo! grew 12 percent. Microsoft, 9.1 percent.

Compounding Microsoft's challenge in the short term is the leadership transition in its Live search business.

Last week, Mary Jo Foley posted a memo from Windows Division President Kevin Johnson saying that he and Steven Sinofsky, senior vice president of Windows and Windows Live engineering, would be naming a new leader for Microsoft's search R&D effort "within the next couple of weeks." That memo was dated March 9, so it's reasonable to expect some news this week or next.