$ curl -i 'http://www.tumblr.com/ HTTP/1.1 302 Found Date: Mon, 13 Feb 2012 19:31:35 GMT Server: Apache P3P: CP="ALL ADM DEV PSAi COM OUR OTRo STP IND ONL" Location: https://www.tumblr.com/ Vary: Accept-Encoding X-Tumblr-Usec: D=15547 Content-Length: 0 Connection: close Content-Type: text/html; charset=UTF-8

$ curl -i 'http://assets.tumblr.com/images/favicon.gif?2 HTTP/1.1 200 OK Server: nginx/0.8.53 Content-Type: image/gif Last-Modified: Fri, 15 Apr 2011 22:13:30 GMT Accept-Ranges: bytes Content-Length: 635 X-Varnish: 1795992965 Cache-Control: max-age=2196224 Expires: Sat, 10 Mar 2012 05:35:30 GMT Date: Mon, 13 Feb 2012 19:31:46 GMT Connection: keep-alive

So it looks like their application is being served by Apache (and PHP), while their static assets are served by nginx behind Varnish.