"Lesson 2: Social Sign-in is Better Than Email Sign-in After dozens of failed A/B tests to improve the conversion rates of our landing page, we came up with the idea to enable social sign-in (i.e., signing up for a Buffer account and logging in using your Twitter, Facebook or LinkedIn account).

After all, Buffer is for posting on Twitter, Facebook and LinkedIn so it only seemed logical that our potential users would be able to quickly and conveniently sign up using one of their existing accounts."

http://sixrevisions.com/user-interface/ux-design-mistakes/