Most sites are more complicated than they seem from the outside. Complexity is fractal -- the closer you get to something, the more complex it is.
One of my past employers was Salesforce.com. Seems pretty simple at first glance (and the early implementations were very simple), but then you learn more. Ignoring all of the platform-related stuff, just doing a multi-tenant database at scale turns out to be plenty hard.
The fact remains, they're a couple orders of magnitude away from extreme load.
