Social login is now an essential part of any site which performs user authentication. It does not need to replace the standard form based authentication, quite the contrary, social login complements it. Login with social accounts is a straightforward process and it saves the users a lot of time, as they won't need to fill the whole form. They just sign up with their social account and they can log into the website with just a few clicks.
Facebook is very popular social network with millions of users worldwide and the number of active Facebook users growing day by day. Their platform allows third party websites to use Facebook as registration or login option with just a few clicks. That is the social login and it allows customers to bring their existing social identities and use them to register and log in without creating a new account explicitly. It saves a lot of time for users and makes the life easier as we don't need to remember all those credentials for different websites anymore.