According to stats company comScore, the top USA sites in July 2011 by visits were Google sites, Yahoo sites, Microsoft, Facebook, AOL, and Amazon. comScore also released statistics for the biggest gainers and the industry with the biggest gains in its full PDF report.

I have included a few stats tables below, and you can download the full report from:
http://www.comscore.com/Press_Events..._for_July_2011