Hey @shyamali 
Unfortunately this is currently a limitation of the Pre User Registration and Post User Registration action flows on Auth0’s end. These actions are only triggered for Database and Passwordless connections as noted in the documentation here:
I see you’ve already made a comment on this feedback post, so hopefully Auth0 will look into supporting this feature for your usecase in the future.
The best you can do for now is preventing these new social users from logging in by using a Post-Login Action. This can be achieved by looking at the event.stats.logins_count attribute of the event and using the built-in api.access.deny function.