Hi nicoloaversa! Try this:
<?php
// get current author object
$curauth = (isset($_GET['author_name'])) ? get_user_by('slug', $author_name) : get_userdata(intval($author));
$author_id = $curauth->ID;
$user_facebook = get_field('user-facebook', 'user_'.$author_id); // see https://www.advancedcustomfields.com/resources/how-to-get-values-from-a-user/
if( $user_facebook ): ?>
<h5><?php echo $user_facebook; ?></h5>
<?php else: ?>
// do something else
<?php endif; ?>