Thomas Ptacek, commenting on GitHub is My Resume:
A resume has just two purposes: first, to help you get an interview, and second, to get you past an HR hurdle....
More importantly: resumes, in any form, are bad at getting you interviews. A resume comes into play at the earliest part of the recruiting funnel, when the hiring team has the smallest number of cycles to spend on each candidate. Your primary strategy for dealing with recruiting funnels: jump the fucking line. It's never been easier to do this! Ten years ago, you'd have to track down someone who worked with someone who worked for someone at the same company as the hiring team. Today, in tech, you just go search Github for projects your hiring team contributes to and start sending pull requests.
I think this is really smart.
It's really hard to "work on open source". One usually requires a reason. I have a number of software projects on GitHub (as well as commits to others' projects) but primarily they were motivated by need. I needed a way to download email, so I maintained a project do that. Now, I don't need that any more, so it has stagnated.
But if you're trying to get a job at a company, contributing to the projects they use makes a lot of sense.