The bad news just doesn't seem to end for Yahoo. As the Internet portal prepares to fend off a proxy fight by billionaire investor Carl Icahn, new research from comScore concludes that for the first time, Google has overtaken Yahoo as the most-visited Web site in the U.S.

Google reached the pinnacle of the Top 50 U.S. Properties in April, with an audience of 141 million visitors, according to data from comScore's monthly analysis. In April, more people used Google's job search, career resources, and television sites, according to comScore.

Yahoo sites ranked second, with 140.6 million visitors, followed by Microsoft sites with 121.2 million visitors. Superpages.com Network, the online directory for local businesses, and jobs Web site CareerBuilder both jumped ahead eight spots to come in 18th and 30th, respectively, according to comScore.

"April was a very active month. Google took the top property position, thanks to continued search growth and rapid growth at YouTube," said Jack Flanagan, executive vice president of comScore Media Metrix, in a statement. "The return of the network television programs after the writers strike sent fans to a variety of sites to get information on their favorite shows, catch up on episodes they may have missed, get refreshed on plotlines, and to stream new episodes."

As U.S. citizens raced to meet the April 15 deadline to file their taxes, a number of tax-related Web sites recorded increased numbers of visitors. For example, 23.4 million visitors logged on to IRS.gov, up 35 percent from March and Intuit.com 's traffic rose 11 percent from March to 13.3 million visitors.

"The significant traffic growth at tax sites, which began in January, reached a crescendo by the tax filing deadline on April 15 as taxpayers sought information, forms, and online filing facilities," Flanagan said in the statement.