They moved away from client-side rendering. Quoting:
There are a variety of options for improving the performance of our JavaScript, but we wanted to do even better. We took the execution of JavaScript completely out of our render path. By rendering our page content on the server and deferring all JavaScript execution until well after that content has been rendered, we’ve dropped the time to first Tweet to one-fifth of what it was.
I'm not a Twitter historian, so I may be wrong, but I'd be willing to bet that their client side rendering allowed them to scale the way they did. When your customers' computers are doing half the work or more, adding a customer costs you half as much or less.
They moved away from client-side rendering. Quoting:
There are a variety of options for improving the performance of our JavaScript, but we wanted to do even better. We took the execution of JavaScript completely out of our render path. By rendering our page content on the server and deferring all JavaScript execution until well after that content has been rendered, we’ve dropped the time to first Tweet to one-fifth of what it was.