One company I work for rejected a candidate based on Github - he'd cloned some repos and stamped his name on it, and claimed they were his. The hiring tech smelled something fishy, and the candidate said that he worked with these various other repos on the backend. Hiring tech reached out to the owners of several repos, and none of them had heard of the candidate before.

Github was used in this hiring process to reveal a fraud - a different 'success' story to others commenting here, but it definitely birthed 'a meaningful question' or two.