When someone logs in with Facebook -

Absolutely. When using the automatic registration feature, the user's Facebook, Google or LinkedIn email address would be stored in the standard Joomla User component and tables.

If you disable automatic registration, the user will go through a form-based flow where you can (optionally) let them edit or enter their email address.

Twitter doesn't have a method to retrieve the user's email address, so they will always go through the form-based flow.

I hope that helps explain, but if you have any questions, just let me know.

Thanks,
Alex