After their infamous "Fail Whale" outages, Twitter replaced its Ruby on Rails / MySQL stack with modern (at the time) technologies: Scala (functional language running on the JVM) / Finagle (RPC) / Thrift (framework) / Kafka (message queues) and this made Twitter stable under the load of several hundred million users.

Ruby is single-threaded, slow with poor GC and memory leaks. As Twitter's Alex Payne said publicly in 2008: "there shouldn't be doubt in anybody's mind that Ruby itself is slow" and that scaling Rails meant "just throw more instances at it"