Research from online analysts Hitwise showed Facebook accounted for 7.07% of internet visits in the US during that week.
During the same week, Google commanded 7.03% of all internet visits, which made both websites accountable for 14% of all US internet visits.
It marks the first time Facebook has been the most visited website across an entire week. The social networking site had previously usurped Google in the US on Christmas Eve and Christmas Day 2009, as well as New Year's Day 2010 and the weekend of 6 and 7 March 2010.
While Google's proportion of visits has remained relatively steady over the last year, building from 6.46%, Facebook has seen an increase in share of internet visits from around a 2.48% share (the fifth most-visited site) in the week ending 14 March 2009, to last week's 7.07% share.
Yahoo Mail accounted for the third largest share of visits in the US in the week to 13 March with 3.8%, followed by Yahoo with 3.67%.
Google's video-sharing site YouTube was the fifth most-visited site in the US with a 2.14% share, and News Corp's social network MySpace had a 2% share of internet visits.
Microsoft's search engine Bing notched up the 10th largest share of US visits with 1.09%, behind Microsoft's news and lifestyle site MSN (1.82%) at seventh place, and email service Windows Live Mail at eighth (1.63%).