Many people recognize that it's much cheaper to place nginx in front of other web servers than to buy more servers. We could say that nginx is mainstream now, so any serious website would use it. If you take "top million websites" instead of "top 1000", you might see different percentages. Automated tests just see nginx in the header, although something else is the actual workhorse for the site.
Some of the stuff does not need Apache anymore. As I moved almost all my projects from PHP to Node.js, there's little use for Apache now. And nginx serves static content with less CPU and RAM.
No reason to use Apache for PHP - I run all of my PHP sites on nginx with php-fpm or fastcgi. It has been much easier for me to manage performance, especially on low-memory VPSs with php running in a separate process. I suppose it would work just as well with Apache behind nginx, but I don't see any good reason to have it in the way unless you rely on htaccess files.
What kind of opcode cache do you employ? Depending on how you set up APC for example, php-fpmcan use much more memory than running Apache mod_php with one shared segment.
Many people recognize that it's much cheaper to place nginx in front of other web servers than to buy more servers. We could say that nginx is mainstream now, so any serious website would use it. If you take "top million websites" instead of "top 1000", you might see different percentages. Automated tests just see nginx in the header, although something else is the actual workhorse for the site.
Some of the stuff does not need Apache anymore. As I moved almost all my projects from PHP to Node.js, there's little use for Apache now. And nginx serves static content with less CPU and RAM.