People keep saying this but I can't really find much anything social about GitHub.

You can follow, star, favourite and comment on things, you get a feed where recent updates on stuff/people you've interacted are listed, you can customise your profile page with snippets about yourself, a photo, a status, contact info and add whatever else you want (including more photos, images, charts etc) in markdown. It now has discussions which are essentially a forum.

It's as much a social network/collaboration tool as it is place to store your code these days.

Some people spend most of their time in issues and PRs, which are social features mapping social interactions.