Very much a social network. Look at your profile/activity feed and how it gamifies contributions with the activity chart (the more you do across github, the more it lights up). Github project stars are just the same as FB likes. Or how they prominently put fork buttons on every repo that immediately clone and copy a fork of the project to your repo. Or that github has an entirely new inbox of messages and issue replies to deal with (and all the issues of potential harassment, etc. that come with that honestly).