There have been a lot of speculation and analysis about the challenges that this type of site will face but I think twitter's problems are much simpler. It was built by a group of people who had never scaled a site to that level before. They've been trying to cope with their success but it sounds like they don't have the right people to write scalable software and don't have enough operations people to keep their systems up.

I attended Railsconf 2007 and two of twitter's developers held an impromptu session to discuss the scaling challenges they faced. Twitter had just experienced several days of downtime and the room was filled with people interested in hearing all about it. It was pretty obvious from that one hour session that their issues are more than just technical.

At one point they showed some code that was used to balance some of their load among a group of servers. Unfortunately they wrote the function in such a way that it was limited to a maximum of 10 servers. Once they exceed the capacity of those 10 servers the whole site came crashing down. They seemed like nice guys and were excited about their work but it was obvious based on their comments and the crowd's reaction that they were out of their league. I got the impression that they are doing whatever it takes to keep the site up and running today, even if it brings the site down tomorrow.

Slides from the session I referenced: http://www.slideshare.net/al3x/scaling-twitter-railsconf-200...