The Internet has become more and more “social” today. Instead of being one–‐way communication medium disseminating information, the sites today wants to engage with the viewers in a social relationship. This helps website owner to know their users better, so that they can target their wares more precisely. This also works as an Internet “word of mouth” marketing tool. So after you land and explore a website for a while, you are likely to be asked to register using one of your existing social network or email identities. This is known as social login.
Read more about social login here