Xenforo Social integrations as Facebook Login and Google Login, were the worst part of whole xenforo installation period.

Even I successfully finish integration, like creating API, getting Api codes and secrets I faced a terrible warning that says:

This account's email is already associated with another member. Please log into that account to associate this account.

In fact I could not find any clear solution from google for that but I saw that I have allready created an user with my facebook or gmail e-mails.
First, be sure that you should not use your social e-mail with another user.

When you clarify it to get rid of this; problem will be more clear:

Just login to the account that you want to solve this problem:

Click Connected Accounts as shown below:





Than what you need is just associate Facebook or Google accounts with Xenforo account.

As shown below:




Xenforo will ask your forum password for connection.

If you do not remember just click Forgot your password? Link in login areas as shown below:

PS: After log out you have to wait about 90 seconds for password recovery.