https://www.quantcast.com/top-sites/US/1
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[0] 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.[1]
[0] http://cl.ly/image/081G1r2f0Z18 [1] http://wordpress.org/extend/plugins/batcache/