graeme_p - 8:46 am on Mar 16, 2010 (gmt 0)
It looks like the number of visits (not hits, not uniques) to the site, by people in the US ony. so it is a fair comparison.
This snippet from the press release is also interesting:
Facebook.com and Google.com accounted for 14% of all US Internet visits last week.
isits to Google.com increased 9% during the same time frame.