The count of contributions (summary of Pull Requests, opened issues and commits) to public repos at GitHub.com from Fri, 19 Jun 2015 15:17:38 GMT till Sun, 19 Jun 2016 15:17:38 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 > 635)