I manage a site that gets about 50 million hits a month. It's a Wordpress site, load balanced over 6 servers, and has a varnish caching system setup.
Right now, 95 - 97% of the time, page views hit the cache. The only time it serves up a new page from the server is when a new story is created, or someone is logged in looking at the stories and commenting.
What I am trying to figure out is that if I move over to Facebook Comments or Disqus commenting, and get rid of the users entirely, will that lighten the load? I would think it would because the only time you would be hitting the server, and not the cache, is when you're logged in - which only the admins would be.
I know it's only 2.5 - 3% but I wasn't 100% sure.