Well, their assets are begin served from a CDN. Are they counting that in their server count?

  $ host assets.tumblr.com
  assets.tumblr.com is an alias for
  assets.tumblr.com.edgesuite.net.
  assets.tumblr.com.edgesuite.net is an alias for
  a1092.g.akamai.net.
  a1092.g.akamai.net has address 69.31.106.32
  a1092.g.akamai.net has address 69.31.106.50