Web traffic analytics firm Experian Hitwise reports that Facebook was the most visited website in the U.S. during the week ending March 13, surpassing Google in what has become an increasingly frequent occurrence in recent months.

Heather Dougherty, Hitwise director of research, writes on the company’s blog that "Facebook.com recently reached the number one ranking on Christmas Eve, Christmas Day, and New Year’s Day as well as the weekend of March 6 and 7. The market share of visits to Facebook.com increased 185 percent last week as compared to the same week in 2009, while visits to Google.com increased 9 percent during the same time frame."

Search Engine Land’s Matt McGee notes, however, that these statistics only compare visits to the two sites’ homepages, and do not take into account visits to subsidiary sites like Google News or Gmail.

The numbers should prompt additional urgency from search engine optimization (SEO) professionals attempting to integrate real-time and social media content into their SEO campaigns. They also demonstrate Facebook’s critical importance as a source of SEO-ready content.