Nowadays, social media has become so popular that most of the internet users have account on atleast one of the social media networks and good thing is that we can allow drupal site users to login/register with their existing social media accounts which saves user's time and makes the registration/login process extremely easy.
Lets have a look at the list of modules that can be used for this purpose.
HybridAuth Social Login integrates HybridAuth library into Drupal and allows your users to login and register using Facebook, Twitter, LinkedIn, Google, Yahoo, Windows Live, Foursquare, AOL, OpenID, Github, LastFM, PayPal, Vimeo, Disqus, Instagram, Tumblr, Vkontakte, Mail.ru, Yandex, Odnoklassniki, Twitch.tv, Steam, etc
Social Login is a free Drupal module that allows your users to register and login to your Drupal website with their existing accounts from 30+ social networks like for example Twitter, Facebook, LinkedIn, Google, Instagram and Yahoo.
This module relies on third pary service called "Login Radius". With this module, users can register by logging in with social ID such as Gmail, Twitter, Facebook etc and they will not require usernames/passwords.
Unlike above modules, Facebook OAuth (FBOAuth) module as the name suggests, allows users to register/login with their Facebook account only.
Similar to Facebook OAuth (FBOAuth) module, Twitter module allows users to register/login with their twitter account only.