I'm guessing your estimates are correct enough. I doubt the user count for all YC projects exceeds 500,000, and according to http://forevergeek.com/articles/debunking_the_myspace_myth_o..., MySpace has at least 43 million, which by your math would mean that Facebook has around 14 million users. Having been a frequent MySpace user myself in the not-too-distant past (I'm 22, and I used it from age 18-20), I concur that there have been times in which MySpace "limped along". It was not uncommon to get multiple "application error" (for lack of a better term -- I don't know/remember what it's called under .NET) during a surfing session, or to encounter a page or profile that just would not load. I've since switched to Facebook, and in comparison, I've never had Facebook barf on me. This, to me, evidences either the dim-wittedness of MySpace's developers, or superiority of Facebook's web solution (http://article.gmane.org/gmane.comp.web.cache.memcached/3224) over what MySpace has chosen.

Note that I am not saying that PHP > .NET -- obviously on sites of this large scale, as has already been noted in the post's comments, using any technology "out of the box" is looking for trouble. What I mean to imply, however, is that perhaps for sites this big, .NET is the wrong thing. And, as the parent comment points, out, IIS certainly isn't helping things, though I don't think Apache would necessarily be a magic bullet. Craigslist, which does many megabytes of data transfer per minute, is considering switching to Lighttpd because Apache too heavyweight.

P.S. Sorry about the links -- I couldn't figure out how to link a text string to a URL. Can someone enlighten me?