You perhaps imagine that when Twitter was created the developers sat down and said "should we write this in Rails or in Java?" I very much doubt a conversation

You perhaps imagine that when Twitter was created the developers sat down and said "should we write this in Rails or in Java?" I very much doubt a conversation anything like that took place. I'm almost 100% certain the developers used the tool they knew best and never considered any other alternatives. Quite possibly they had little or no experience with alternatives. I work with both a RoR code base and a Scala code base. There is no difference in productivity that I can tell between the two. I think it is a out-dated assumption to believe that dynamic languages like Ruby or Python are more productive than modern statically typed languages. The only cost is one of learning; many more people know Ruby than Scala. This is an artifact of history (i.e. Ruby is 10+ years older than Scala; crappy CS educations that only teach Java).