I wonder this as well, especially with "top" websites. How much high profile traffic these days is actually served by a single piece of software?
All of our high profile sites have layers of caching and load balancing between them and the Apache backends that actually generate the content. In our case Nginx is in fact on the public facing end performing reverse proxy and TLS termination only. That is not to say Nginx isn't a fantastic product, but it may not be entirely fair to Apache, Varnish, and friends to measure HTTP server marketshare in this way.
All of our high profile sites have layers of caching and load balancing between them and the Apache backends that actually generate the content. In our case Nginx is in fact on the public facing end performing reverse proxy and TLS termination only. That is not to say Nginx isn't a fantastic product, but it may not be entirely fair to Apache, Varnish, and friends to measure HTTP server marketshare in this way.