Facebook.com grew to 26.6 million unique visitors in the U.S. in May 2007, marking an 89 percent increase compared with May 2006, according to a comScore study on visits to the social-networking site, writes MarketingCharts.

Facebook in Sept. 2006 decided to open registration to the general public, rather than confining itself to colleges and other schools.

In the months prior to allowing open registration, Facebook.com's traffic hovered at approximately 14 million visitors per month. After Sept. 2006, visits accelerated to 26.6 million visitors in May 2007, up 100 percent from Sept. 2006.

Moreover, the number of pages of content viewed at Facebook.com in May 2007 increased to 15.8 billion, up 143 percent from May 2006 and 121 percent from September 2006.

Visitors averaged 186 minutes at the site in May 2007 - a 35 percent increase from a year earlier. Engagement levels peaked in February at 200 minutes per visitor, then leveled off slightly as the influx of new visitors (who tend to begin as lighter users) tempered the average.

See MarketingCharts for comScore data on the demographic profile of all those new visitors.