It's up to you to handle that.
http://laravel.com/docs/5.1/authentication#social-authentication
Socialite just provides an easy way to authenticate with Facebook and get the details. You can store the details in your users table.
// All Providers
$user->getId();
$user->getNickname();
$user->getName();
$user->getEmail();
$user->getAvatar();
$user->getId() would be the unique id returned from Facebook. Save this field somewhere in your database, either in a separate providers table, or even the users table would be fine. Now when someone tries to login with Facebook, if that ID doesn't exist, either create a new account for them or throw an error.
Just remember, the ID returned is from Facebook not your webapp. So your user would have an "id" for your web app and a "facebook_id" that you store so you know what user they belong to.