You may enjoy to read about the architecture [1] behind the highly successful LinkedIn services used by millions of people. Dionysios G. Synodinos at InfoQ has been talking [1] to the technical people behind it all, Alejandro Crosa, Chris Conrad and Jay Kreps. They describe the architectural decisions, trade-offs and the benefits of bringing Scala into the equation and how it all plays together with JRuby and back-end databases. Alejandro is clear about Scala, "If you use Scala just like you would use Java then you are just getting a more succinct syntax and nothing else. Embrace immutability, functional data structures, pattern matching, Actors, etc. and you'll never go back."
Links:
[1] http://www.infoq.com/articles/linkedin-scala-jruby-voldemort