So I registered on Stack Overflow the other day, and I was asked if I wanted to register with an already existing account. If I recall correctly the options were Facebook, Gmail and something else I forgot. So I chose Gmail, and it took 200% less time to complete the registration, which I find astonishing. Can you guys please share how you achieved this, the code you use and if you had to speak to those companies (Gmail, Facebook) in order for them to let you implement this system?
I am very curious, and every info is much welcomed and appreciated.