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.
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