"I bet Rails programmers are not happy"
I couldn't care less... Twitter is overhyped and flawed from the beginning. Yeah, I use it, but could definitely live without it.

"#1 success application at a large scale"
Did you mean "successful"? "#1"? Wrong, e.g. both yellowpages.com and scribd.com are huge and serve a lot of requests everyday (very close to twitter's numbers) and I think they'd never experienced problems.

I can name a few of Twitter's deadly sins:
1. Using Rails to serve api requests (said to be 90% of their traffic). They should be using Merb or any other framework with a smaller footprint on requests where they don't need to mess with forms or templates. It's plain XML or JSON, guys...
2. Cook says on his "Scaling Twitter" <a href="http://www.slideshare.net/Blaine/scaling-twitter">talk</a>: "Index every column that appears on a WHERE clause" and that's just plain stupid. That means that when you write a new line to a table, the database will rebuild the index for "every column that appears on a WHERE clause".
3. He also says "Denormalize a lot". Anyone that knows ActiveRecord and can read Ruby must gasp at his 25th <a href="http://takk.webreakstuff.com/~tpinto/pics/denormalize.png">slide</a>.
So, this guy creates a "text" column in his table to be filled out with CSV that will parse later with Ruby. He removes complexity from the database (that would be a lot lighter on it) and messes with it on Ruby.

i.e. for searches, I bet they are using plain SQL using their "index everything" approach. Maybe they should be using <a href="http://rubyforge.org/projects/ferret/">ferret</a> (just like lucene for Ruby) to index and search all those messages and leave the database alone.

Seriously, just with everything else, with Ruby and Rails you just need the right people. Maybe Cook should've been fired sooner.