Add FontAwesome social links in an li in your primary WordPress nav.

wp-append-social-links.php

/**
 * Add FontAwesome social links in an <li> in primary nav location.
 */
add_filter( 'wp_nav_menu_items', 'rose_append_social', 10, 2 );
function rose_append_social ( $items, $args ) {
  if ($args->theme_location == 'primary') {
    $items .= '<li>
      <a href="https://www.facebook.com/YOUR-FACEBOOK" target="_blank"><i class="fa fa-facebook-square"></i></a>
      <a href="https://twitter.com/YOUR-TWITTER" target="_blank"><i class="fa fa-twitter"></i></a>
      <a href="https://www.youtube.com/user/YOUR-YOUTUBE" target="_blank"><i class="fa fa-youtube-play"></i></a>
      <a href="http://pinterest.com/YOUR-PINTEREST" target="_blank"><i class="fa fa-pinterest"></i></a>
    </li>';
  }
  return $items;
}