But that only includes *.wordpress.com subdomains, and our highest traffic blogs almost always invest the money to have their own domain. We have a tag to track those in Quantcast, and it's currently at 129.7M people in the US, which would place it between Facebook (143M) and MSN (98M). (Blogger might have a similar boost into the top 10, but I don't see any others in the top 50 that could have so many mapped domains.)
Of course we cache, with a publicly available WP plugin called Batcache.
I agree that using caching is normal, but the difference is the total reliance on caching. I wouldn't be impressed with google's scalability either if it were 99.9999% static pages being served up.
I think we agree that "scaling" small (sub-RAM-size) amounts of data to a largely logged-out and cached audience is easy, but I think you think of WordPress.com as much smaller and more static than it actually is. My apologies if I'm misunderstanding your point of view.
If you want to see standalone sites in the top 100 running WP besides wordpress.com, check out time.com, umbrellanews.com, celebuzz.com, and large sections of nytimes.com, cnn.com, and people.com. If you were to spider the top million Alexa domains, you'd find about 17% of them on WP:
I realize lots of people run wordpress. That doesn't support the claim that it easily scales to the traffic demands of a top 10 site.