> How many questions they’re asking on StackOverflow

StackOverflow is no more valid a metric than GitHub. Off the top of my head, Pascal, and all its dialects and variants, comes to mind. Many Pascal users and those of the many dialects that it has, just about never use StackOverFlow. They can use websites that correspond to their dialect or variant (Delphi, Object Pascal, Free Pascal, Oxygene...), books (which there are many), etc...

But, for the newer languages, I do think GitHub is much more representational. Not just the stars, but adding other GitHub metrics, can give a more wholesome picture.

I do think that each language, along with the ecosystem and culture that surrounds it, has to be examined by using various metrics and different sources in order to get a more complete picture. GitHub metrics are telling us a lot, just not the entire picture.