This is true. Though I can say that if one were building twitter and you didnt either have a design like the one I suggested, or a design with some kind of distributed caching then you are going to have exactly the kinds of scaling problems they have had. In other words if you use tradition techniques for something like twitter you are screwed. However you are absolutely right that (as I said in my piece) they may already, for example have thought of all of this and either implemented or dismissed it because they have a better design, and I have no way of knowing.

This isn't true. You don't need a distributed sharded setup in order to scale. (You probably do if you are limiting yourself to MySQL) I built financial applications way back in the day that had a lot more reads and writes than twitter, and we just used ONE gigantic, super expensive database server.