Scalability is an architecture issue, not a programming language issue. You can certainly build linearly scalable system in Ruby. It just costs more to scale a Ruby app vs. an equivalent Java app. In the end, the tradeoff is between engineering cost and operating cost. When you spend 100k to hire an engineer and your traffic can be handled by a few boxes. Engineering productivity is your primary focus. As your traffic grows, you need more and more servers, server efficiency becomes more and more important. At some point, the savings in server cost justifies the increased cost in engineering effort and you make a switch from Ruby to Java.