Stack Overflow is geared towards developers and doesn't have anything I can see to attract a particular kind of developer, except the curious kind. I can't think of a place to get a better sampling of developers right off hand. Certainly, while Hacker News is popular among developers, it would be a less accurate sampling of developers than Stack Overflow, because people here tend to be attracted to startups.
I disagree with what you said in your first post, that it is completely wrong, and that no valid conclusion that can be taken from these numbers. It's not perfect but it's far from being a terrible sample. It's a general-interest developer site rather than a specific-interest one. There are all types of developers. In a reverse-sorted list of popular recent tags, there are c#, javascript, php, java, jquery, .net, and android. Also, developers didn't choose to be a part of this graph; they merely got put into the survey result because they had Stack Overflow accounts. If it had been a survey that was announced on twitter it would be biased towards people who want to take surveys. This would be a worse bias IMO than people who want to ask and answer technical questions.
I wouldn't take issue if you had said it was problematic, but instead you went straight to a one-sided conclusion.
