Automattic to be consistent with their brand want an all opensource stack.
Nginx is super fast, reliable, and stable. You can squeeze a lot more juice out of a server with Nginx than you can with Apache. Using Nginx is almost like having two servers running Apache – except, faster, more reliable and stable.
According to Quantcast, WordPress.com’s network traffic including its subdomains, and hosted domains make it the 6th most visited site.
A quick analysis shows WordPress.com using Nginx 0.6.29.