Google has overtaken Yahoo to become the number-one network of sites on the web.

Data from research firm comScore reveals that Google and its affiliated sites recorded more than 141 million unique US visitors in April.

Yahoo finished just shy of its longtime rival, logging roughly 140.6 million visitors over the course of the month.

Microsoft's family of sites came third with 121 million, followed by AOL with 111 million. Fox Interactive Media rounded out the top five, logging 87.5 million unique visitors last month.

Yahoo retained its advertising lead over Google with 160 million unique visitors, ahead of Google's 155 million. Both companies were behind advertising networks Platform-A and Advertising.com.

Sites with the greatest upswing in visits included music site Playlist.com and Major League Baseball, which saw its season kick off in early April. The mid-April tax deadline pushed IRS.gov into the number three spot.

"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's Media Metrix.

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

"The return of network television programmes 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 stream new episodes."