like V-2's top-level comment explains, there are lots of ways to interpret these statistics and it's easy to jump to false conclusions if you don't take the time to look for context.
In Ruby's case, the total number of repositories on Github has continually increased -- it's just that since it was such a huge part of Github's early user base (the Rails community was probably the first big adopter of Github, which makes sense since Github itself is written in Rails) percentage-wise it has dropped significantly as more communities adopt Github.
