By mid-2008, one of these Ruby message queues completely crashed and developers needed two and a half hours to shove the dropped Tweets back through the system. When your game is micro-blogging, that's a lifetime. Then, in his spare time, one developer ported the code to Scala. According to Payne, the Scala queue could process the same message backlog in 20 seconds.

By the end of this year, Payne said, Twitter hopes to have its entire middleware infrastructure and its APIs ported to the new language. Ruby will remain, but only on the front end. "We're still happy with Rails for building user facing features... performance-wise, it's fine for people clicking around web pages. It's the heavy lifting, asynchronous processing type of stuff that we've moved away from."

For this sort of heavy lifting, languages like Java and C++ may be as fast as Scala. But Payne wants more than speed. He prefers Scala because it combines functional and object-oriented techniques. "Functions are objects, and objects are functions," he said. "Scala asks can you have the best of these two worlds together? And you can."

"The biggest selling point for certain organizations is that Scala is 100 per cent compatible with your existing JVM code. There's no wrappers. There's no bullshit. You just load up your JVM code and call in to it, and there's no performance penalty. If you have a big investment in Java infrastructure, Scala is the ideal language."

Excerpt taken from:
http://www.theregister.co.uk/2009/04/01/twitter_on_scala/

More on Scala:
http://www.scala-lang.org/