I don't think this is a "social graph play". I think it's about them realizing that the small amount of revenue they get from individual subscriptions is worth

I don't think this is a "social graph play". I think it's about them realizing that the small amount of revenue they get from individual subscriptions is worth less than the free advertising/marketing they get by making it free for individuals, and then letting developers advocate for GH inside organizations which end up buying Enterprise licenses. This is more about getting people to use (or keep using) GitHub for their personal projects so that it seems like a natural/obvious/default choice for a bigger-budget project, and that's where the revenue is.