There are lots of sites powered by Drupal. But how to find them?
One way is though google. Searching for inurl:"user/login"
finds about 3,730,000 results, most of them are probably Drupal. Which of them are in English? Searching for inurl:"user/login" AND intitle:"User account"
returns 1,030,000 results, which means more then a million are in English.
Which are in the USA? Google Doesn't tell. To get more meaningful results, use TrandsPro data, download the Alexa top 1,000,000 list and order your own custom crowler. (Here is how to write one.
TrendsPro reports about 555487 sites using Drupal, 109990 (19.8%!!) of them are in the list of most influencal sites on the web. In comparison, this list contains 8417245 sites using wordpress, but only 1120592 (13%) of them are in the list of most influencal sites in the web. So if you build a site with Drupal, you have a better chance to be on the TrandsPro top sites list then if you use wordpress.
TrendsPro provides lots of other intresting statistics, essential for any IT decision maker, so I highly recommend registring to their free account.
Photo source: http://trends.builtwith.com/cms