I don't see the parallel between github and Stack Overflow. I can understand why programmers in certain industries and companies won't be posting their code on github. If all of the code you write professionally is proprietary, it makes sense that none of it would wind up in github public repositories. But even if I'm writing proprietary code, I can still ask questions about it on Stack Overflow, and answer other people's questions.
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I don't see the parallel between github and Stack Overflow. I can understand why programmers in certain industries and companies won't be posting their code on github. If all of the code you write professionally is proprietary, it makes sense that none of it would wind up in github public repositories. But even if I'm writing proprietary code, I can still ask questions about it on Stack Overflow, and answer other people's questions.
Stackoverflow: 212 Ada followers 508 FORTRAN followers 38,600 C# followers 44,500 JavaScript followers
Github: 329,678 Search language:ada 720,910 Search language:fortran 6,034,816 Search language: c# 44,430,272 Search language:javascript
The two services are heavily used by the C#/web community. mobile too but I did not include numbers