nav_menu_css_class filter to add classes to the social links of the menu

wp_add_fa_classes_menu_social_links.php

<?php

function add_icon_social_links($classes, $item, $args ) {
  if ( 'social-links-menu' === $args->theme_location ) {
    $class = 'fa fas';
    $email_pattern = "/^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,})$/i";

    $host = parse_url($item->url, PHP_URL_HOST);
    switch ($host) {

      case (preg_match($email_pattern, $item->url) ? true : false):
      $class .= ' fa-envelope';
      break;
      case (preg_match('/facebook.com/i', $host) ? true : false):
      $class .= ' fa-facebook';
      break;
      case (preg_match('/twitter.com/i', $host) ? true : false):
      $class .= ' fa-twitter';
      break;
      case (preg_match('/youtube.com/i', $host) ? true : false):
      $class .= ' fa-youtube';
      break;
      case (preg_match('/instagram.com/i', $host) ? true : false):
      $class .= ' fa-instagram';
      break;
      default:
      $class .= ' fa-link';
      break;
    }
    $classes[] = $class;
  }
  return $classes;
}
add_filter('nav_menu_css_class' , 'add_icon_social_links' , 10 , 3);

 ?>