22% of Drupal Sites Are in the List of Most Influential Sites on the Web
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 currently reports about 513194 sites using Drupal, 113644 (22,1%!!) of them are in the list of most influential sites on the web this week, and an additional 399,550 sites using the technology across the rest of the web (an additional 150 million sites). In comparison, this list contains 7500424 sites using wordpress, but only 1257293 (16%) of them are in the list of most influential sites in the web this week. 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.
Source of the image: http://trends.builtwith.com/cms