I know more than a handful of exceptional engineers who don't even have GitHub accounts. And I know a few really average engineers who are active on GitHub. As

I know more than a handful of exceptional engineers who don't even have GitHub accounts. And I know a few really average engineers who are active on GitHub. As best (if it works like magic even), this only gets you the cream of a small subset of engineers. But if this is the only source you would use to hire, you leave out a large pool of talent before you even begin.