While there are many corner cases which you have outlined, in the grand scheme of things job advert data is a reasonable indicator (and one of the few we have). In the case of languages with limited commercial adoption (Clojure, Haskell) there will be more noise due to the small sample size and low adoption. The indeed.com data indicates that Java and C# are popular languages. Do you believe that to not be an accurate reflection of reality?

What is interesting about the blog is that it also correlates the results with results from polls among developers.