You may have read articles explaining Ruby on Rails was the system behind Twitter and how there were numerous issues, for such a demanding and successful service as Twitter. In an article from Technology Review, Alex Payne from Twitter explains how they hope to replace a lot of the back-end systems in Ruby on Rails by Scala based services by the end of the year. Extract to understand what Scala is :
So the Twitter team turned to Scala, a programming language with its origins in work by Martin Odersky, professor at EPFL in Lausanne, Switzerland, around 2003. During his presentation, Payne, who's also writing a book on the language, explained that Scala has many of the benefits of other languages but without the drawbacks. Some of the characteristics that make Scala so appealing to Twitter is that it's able to efficiently handle concurrent processing--that is, separate instructions that need to use the system's resources at the same time. This is useful when messages from millions of people need to be sent out instantly to different devices all over the world.
Although Scala has, like any language, it's weak points, it seems that the language has great advantages for a company like Twitter. It is a leap of faith though for the company, since there are few, to no examples out there comparable to Twitter.