An article has just been posted on the Twitter Inc. blog about the evolution of the Twitter architecture, infrastructure and technology.. For your convenience here is an exerpt ..

"...We hit some scaling stumbling blocks a few months back, but not because Ruby or Rails was working against us. Once Twitter reached a certain amount of traffic we were forced to rethink our architecture; you don't build a messaging system the same way you build a content management system. We set about developing custom solutions both inside and outside our Rails application. We also made good use of web scaling standbys: caching, database optimization, more hardware, and shared experience. Throughout this scramble to scale, Ruby and Rails were assets for their speed of development and creative, helpful communities.

For stressed engineers, it's tempting to think that another solution - anything but what you're using now! - will solve all your problems. Maybe you start dreaming up the perfect framework on a whiteboard, or maybe you start scouring the web for the fastest, newest, most experimental technology. In the long run, picking a foundation you're comfortable with and making sensible iterations towards your performance goals will yield a bigger win. You have to be careful while iterating: watch your database, test thoroughly, and be ready to roll back when things break (and they will)..."

Go here to read the full article. .