I think this tells us as much about GitHub as it does about changes in programming language popularity. Ruby's decline is the most stark example because GitHub itself was one of the early Rails successes, Rails was hosted there and many other Ruby projects moved there early on as well. So its percentage of MAU was higher simply due to this early network effect.As GitHub has grown the percentage of its users that use Ruby is dropping.<p>I suspect Java is still more popular than Python, but much of that code is private and not on GitHub anyway.