Not to brag, but 3M is by far not one of the largest Drupal powered sites, just one property of the company i work for has more than 40M active users a month & 350M pageviews
I never claimed it was the largest. Just one of the larger ones, particularly as it has a community of 200k registered users all using community forums (most Drupal sites are more in the 100s to 1000s of users). It's the largest I know of offhand managed entirely by a single person and not using any specialized front-end caching or load balancing between the actual Drupal instance and the end user. There are much larger sites run by large companies with all kinds of specialized code and handlers like The Onion, Weather.com, etc.