Who says Drupal doesn't scale? Certainly not Acquia!
A large global PR firm recently measured their Acquia-hosted web site performance to over 3 million page views per hour (or 830 pages per second). When including all content, that's 5,000 requests per second. Wow!
Drupal gave them the flexibility to quickly respond to events. Of course, the bottom line is customer success and they reported that it was one of their "best web launches... ever".