Dries is the founder and project lead of Drupal and co-founder of Acquia and Mollom.
Strayer University, with more than 44,000 students enrolled at over 70 campuses, is using Drupal on http://strayer.edu.
They've been using Drupal for a couple years now. I was the lead developer on their Alumni site - http://alumni.strayer.edu/ - just over two years ago. The most difficult part at the time was making sure they could validate new users against their existing user/alumni database.
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