Hi
I have the same problem and I was very grateful for getting to know the solution.
How to register / login through facebook twitter or google ?

Facebook registration

1. Need a facebook account to access facebook developer and create an application
2. Record application id and secret key from newly made application
3. Select website login on the list
4. Enter site URL

On the joomla site
1. Enable facebook login plugin, fill up id and secret key.
2. Enable frontend login.

Worth trying.