comScore August data showed that Facebook inches past Google in terms of time spent online. "People spent total 41.1 million minutes on Facebook, about 9.9% of their Web-surfing minutes in the month. In same period, people spent 39.8 million minutes, or 9.6%, on Google's sites combined, including YouTube, Gmail, Google news and other content sites. comScore puts Yahoo in 3rd place, with U.S. web users spending 37.7 million minutes on its sites, which was about 9.1% of their web surfing time in August."
Hitwise has also been reporting that Facebook is the top US site for several weeks at least: