Mr. Pop History – Google began surpassing #1 Yahoo (counting all services) in May of 2008. From mrpopculture.com and the week of May 15, 2008:
Google has surpassed Yahoo to become the most popular website in the U.S., Google has long been the Internet’s leader in search but its audience has trailed Yahoo’s when counting other services such as e-mail and photo sharing. The lead is tiny – 466,000 visitors out of about 141 million apiece.