It's not definitive and it's coming from just one website analytics company, but Facebook is now the biggest site in the US. According to the latest numbers from Hitwise, Facebook squeezed past Google to become the most visited site in the US in the past week. It's a very small gap between the two and other analytics companies aren't seeing the same numbers, but it's still a significant milestone.

"Facebook reached an important milestone for the week ending March 13, 2010 and surpassed Google in the US to become the most visited website for the week. Facebook.com recently reached the #1 ranking on Christmas Eve, Christmas Day, and New Year’s Day as well as the weekend of March 6th and 7th," Hitwise announced.

Year over year, Facebook grew 185 percent in traffic in the last week, a significant rise for a site that already had tens of millions of visitors per month a year ago. In the same time frame, Google only grew by nine percent, understandable, as it was already the biggest site in the US at the time. Facebook and Twitter add up to a little over 14 percent of the entire US web traffic.

Facebook accounted for 7.07 percent of the web traffic in the US last week and Google for 7.03 percent. The two are followed by Yahoo Mail with 3.8 percent, Yahoo with 3.67 percent and YouTube with 2.14 percent. The top five web proprieties in the US account for almost a quarter of all the traffic in the country.

These numbers may not be extremely accurate, but, even if the actual numbers are wrong and Facebook is still at number two, the trend is very, very clear and it is only a matter of time until Facebook overtakes Google for good. Google has enjoyed its position as market leader since September 2005 and, at the time, it surpassed MySpace to take the number-one spot.