Straight from WordCamp Seattle, Scott Berkun on how WordPress.com is made. You can read more on the WordPress.com News blog. (Listen carefully for metrics Scott mentions that you may never have thought about that are available as a result of your using WordPress.com.)
To be fair, Raanan Bar Cohen mentioned how Automattic handles internal communication when he spoke at WordCamp Israel in 2008, when WordPress.com had a mere 4.7 million users. Today, WordPress.com is closing in on 20 million users.
(If the above embed link is not working, you can watch Scott’s presentation on WordPress.tv)