The count of contributions (summary of Pull Requests, opened issues and commits) to public repos at GitHub.com from Tue, 06 Dec 2016 17:06:46 GMT till Wed, 06 Dec 2017 17:06:46 GMT.
Only first 1000 GitHub users according to the count of followers are taken. This is because of limitations of GitHub search. Sorting algo in pseudocode:
githubUsers .filter(user => user.followers > 1000)