Yup, came here to say that. I think this is a great development, since the subresources of github, twitter and mediume have nothing to do with each other (as opposed to other subresources like a normal TLD).

I think this is one of the few aspects where tumblr is more logical than other social networks, by using a subdomain for each user instead of the path.

I think reddit accepts this too to some extent - for example, programming.reddit.com will redirect you to reddit.com/r/programming.