Github issues have also replaced most of my stack overflow usage. Part of it is that I'm now more experienced, so most of my questions are about edge cases or potential bugs and not "How do I call system()?". But it also feels like stackoverflow took off because it was lower friction than mailing lists and experts exchange, but has been overshadowed as projects migrated to github and most developers already having a github account, rendering it lower effort. This changed things by (a) having stuff in google results from other sites and (b) encouraging developers to write better docs to reduce the rate of support requests in github issues.