Thanks to Facebook connect, you have the opportunity to collect information about the user so you can offer him a pre-filled form that he has only to validate after accepting the conditions of participation. You can trace the following information:
E-mail
Name
Last name
If you want to collect more information about the user, you can use a Reach 5 connector.
This will allow you, in addition to knowing the first name, name and email of the user, to know his age, his city and the pages he has liked via his Facebook account.