New figures from research firm Hitwise show that Facebook.com now accounts for 7.07% of all American Web traffic.
Google.com accounts for 7.04 per cent of traffic. The latest Hitwise figures cover March 7 to March 13.
It is the first time that Facebook.com has had a weekly lead over Google.com.
Heather Dougherty said that Facebook had "reached an important milestone" with the latest figures.
Facebook had 400 million users worldwide in February 2010, up from 200 million in April 2009.
The Hitwise numbers are just for Google.com and do not include national sites - such as Google.co.uk - other Google services such as Maps, or Google searches made within a browser toolbar or on other sites.
If all Google properties are taken into account, the company had 11.03% of U.S. Web site visits last week.
According to Nielsen, Internet users worldwide spent an average of more than five and a half hours a month on social networking sites in December 2009 - an increase of 82% on the previous year.