If only things were as simple as they look, we would ever see the Twitter fail-whale. Twitter has two major issues:

1. most of the tweets are robotic, all sorts of updates from other sites, feeds, listing, etc twitted and re-tweeted for no other reason than taking a little extra token of Internet space. Someone uploads an instagram - here comes a tweet, someone posts to freelancer - here's another tweet, etc, etc. What you see/follow/read on twitter is a tip of the iceberg - try search and you'd see more be flooded by spam-tweets that can not really be classified as such, and hence have to be served,

2. like most other "wonder-sites" of today Twitter doesn't spends more money than it makes. This translates into limited hardware capacities, and there's no solution in sight.