Mind you, Twitter may not be the best example. Its original architecture was written in Ruby, at a time when few development houses were willing to gamble on that platform for mission-critical projects. And Twitter's history of repeated outages might lead you to believe that anything would be a worthwhile replacement.
Still, Twitter's example is worth considering. Until venture capitalists, investors, and executive management are willing to accept the guidance of software developers in choosing languages for major projects, languages like Scala are liable to be relegated to the use of a few "passionate" programmers at quirky Internet startups like Twitter -- no matter how much potential they have.
Before development managers can choose from a variety of different languages, however, they need to have coders on their team who understand more than one language. That's why I'd like to see universities graduate more CS students as "polyglot programmers," rather than language specialists. If nothing else, I worry that students who spent most of their education learning the ins and outs of a specific language syntax may be missing the bigger picture. Human language skills will always be the most important tool in a good software developer's belt, but a foundation in sound software design is what helps a good developer become great, no matter what the language.