Google and Facebook top 2011's most visited sites in US
Google was the most popular website with US users in 2011 but Facebook was not far behind, according to market researchers.
Nielsen suggests more than 153 million visitors clicked onto Google branded pages each month, as Facebook attracted close to 138 million visitors.
Yahoo came third with about 130 million visitors each month.
But analysts warned Yahoo's tally might be at risk if young people continued to turn away from web-based email.
The study is based on data collected between January and October and included visits from home and work computers. It involved a sample from a global panel of 200,000 people.
Although Google trumped Facebook as the most popular web brand, the search giant's Google+ network came far behind Mark Zuckerberg's site in Nielsen's ranking of the most popular social networks and blogs.
Google+ came eighth in the list with 8.02m unique monthly visitors.
That also put it behind Google's weblog publishing tool Blogger, as well as Twitter, Wordspace, Myspace, Linkedin and Tumblr.
Google's YouTube was identified as the most popular destination for online videos, attracting more than three times the number of monthly visitors as the music video service Vevo.
While Yahoo maintained its position as one of the top three web brands, an earlier study cast doubt over its ability to retain the position over coming years.
Research by Comscore suggests US-based 12- to 17-year-olds spent 30% less time using web-based email between November 2010 and the same month this year.
Usage among those aged 18 to 24 fell by 21%, and was down by 1% in the 25 to 34 age bracket. Usage was up among older age groups.
Internet analysts said this was a worrying trend.
"Yahoo's basic problem is that people are no longer looking for an all-you-can-eat service and instead want best-in-breed," said Ian Maude from Enders Analysis.
"For social networks that is Facebook, for search it's Google.
"Email is a front door to Yahoo and if people are no longer using their service it will affect them more broadly. If Comscore's data is an early indicator of a growing trend they have a major problem."