The big difference between blog hosting and things like Twitter (or Reddit, or Digg, etc) is that blogs are independent, so adding servers scales you up linearly. When you are looking at X's blog, basically everything you see is coming from one server.

You will have to have something that deals with mapping URLs in the unified logical name space of your site to the individual servers that the particular blogs on--that's the part that you can't just throw machines at and get good results.

With the social sites, you can't isolate things easily, because what a given person sees at any time is drawn from an ever changing set of content from other users, with each viewer drawing from a different set.