According to Hitwise, Facebook was the most visited website of the year. This is if you count Google properties separately.
The combination of Google properties accounted for 9.85 percent of all U.S. visits. Facebook properties accounted for 8.93 percent, and Yahoo! properties accounted for 8.12 percent. The top 10 Websites accounted for 33 percent of all U.S. visits between January and November 2010, an increase of 12 percent versus 2009.
[Hitwise]