If I understand, you would like to add for example Facebook information to a logged-in user.
So what you have to do in your provider is to check that there is a connected user and then add information:
if (($this->container->get('security.context')->getToken() instanceof AbstractToken) {
$loggedUser = $this->container->get('security.context')->getToken()->getUser();
$loggedUser->setFacebookId($fbdata['id']);
/* userManager is fos_user.user_manager service */
$this->userManager->updateUser($loggedUser);
return $loggedUser;
}