add_action( 'facebook_login', 'facebook_login_after', 35, 1 );
function facebook_login_after($form) {
    if('login' == $form) {
        echo '<p style="font-size: 12px;">' . __('Using Social Login to enter the website means acceptance of our <a href="#">T&C</a>', 'userswp') . '</p>';
    }
}