How big is the codebase for twitter? Are you limited by devs or production resources?

You can also swap in Java components when you need to so, depending on the architecture, you may only need a small Scala team. Java devs are everywhere. You also win by acquiring easy access to Java libraries.

Not saying Scala is the right choice... but there are big advantages besides "Scala is lighter than Ruby" -- and Scala is a lot lighter.