What about developers that don't spend much time online? Or that don't find much personal joy in answering the questions of strangers?
Just because Stack Overflow might be one of the best places to get a sampling online doesn't mean it is actually a correct sampling of the population of software developers.
Those are good points and I agree that there are numerous reasons why it isn't a correct sample. What I'm trying to say is that it's hard to get a correct sample for developers and that if someone is curious about the subject matter it's worth keeping this data until some clearly superior data comes along. There are plenty of datasets that are worse than this, either by having fewer data points or by being even more biased.
Just because Stack Overflow might be one of the best places to get a sampling online doesn't mean it is actually a correct sampling of the population of software developers.