Happy to write an article on that. I initially built the website using Ruby on Rails and had twitter/facebook omniauth implemented.
It was then a simple case of displaying a web view in the app with login similar to how you would on the web, then diverting back to the app upon login.
It was then a simple case of displaying a web view in the app with login similar to how you would on the web, then diverting back to the app upon login.