Add social login to existing login system

We have an existing custom user registration system which uses the following fields for authentication/account creation. username password email gender date of birth Upon registration an authentication email is sent to user to verify email. After that they can login with username or email + password combination. I want to add social logins (primarily facebook/twitter/google) to complement the existing system. The 3 case scenarios should apply Account exists (created using custom registration process) 1) Logged in and then does social login. Look up email and add social credentials to existing account 2) Not logged in but does social login. Look up email. If found ask if want to create new account or merge with existing account New account: 3) Logs in via social login and no account exists, then create new account. Optionally send verification email to verify social login email and also set a temporary password to enable user to login using email/password combination. Also assigns username (unique) and checks to see if any user exists with that username. This would be phase 1. In second phase, we need to integrate tools to allow social sharing of content from within the app and also track and monitor the content shared (likes, +1's etc) Immediate work. The site url will be shared after signing NDA.