Somehow I do not feel confidant about correlating language popularity on stackoverflow to the actual popularity. For some languages (e.g. in my observation:, c#, objective c), there are only few tutorial/sample code outside of stackoverflow as compared to other languages (e.g. again in my observation Java). This makes more and more activity on stackoverflow for some languages as compared to the other languages which has more sample/tutorial on other place on internet.
In my observation, whenever I am writing code in c# and objective c, I almost always land to stackoverflow by google search but this is not the case whenever I write code in JAVA