I think people contributing to large well known projects on Github is a fantastic way to find good engineers. Anyone can throw up a simple application on their own github, but it takes a myriad of skills to successfully contribute to the aforementioned type projects.
You have to dive into an unfamiliar codebase you didn't write, identify a problem or missing feature, write out an issue, discuss it with a group of people, send a PR, write unit tests, follow a style guide, discuss inline on implementation, and more.
This is a lot closer to real world software development (at least at most larger or medium sized companies) than just writing a toy demo of something.