The count of contributions (summary of Pull Requests, opened issues and commits) to public repos at GitHub.com from Mon, 01 Dec 2014 00:48:25 GMT till Tue, 01 Dec 2015 00:48:25 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 > 508)