Interesting point-of-view. I never use Github/Google/whatever to sign up with another website in an effort to minimise their cross-site tracking. In fact if a site only offers FB/Goog/Github sign-in and lacks their own sign-in identity management, I simply refuse to use it at all and go elsewhere. (And there's always an 'elsewhere'!)
Sorry about that! Wasn't the first time I've heard this suggestion -- I'll definitely look into supporting this.
Your site says it’s written in Ruby. Check out the OmniAuth gem for adding login providers. It’s probably worth a quick look at similar gems before choosing this one.