Getting all that facebook comment malarkey off into a tab, that only appears when the field_facebook_comment field on the node is true.

social_commentary_js_facebook-comment.js

(function ($) {
  Drupal.behaviors.facebook_comment = Drupal.behaviors.facebook_comment || {
      attach: function (context, settings) {
        (function (d, s, id) {
          var js, fjs = d.getElementsByTagName(s)[0];
          if (d.getElementById(id)) return;
          js = d.createElement(s);
          js.id = id;
          js.src = "//connect.facebook.net/en_GB/sdk.js#xfbml=1&version=v2.8";
          fjs.parentNode.insertBefore(js, fjs);
        }(document, 'script', 'facebook-jssdk'));
      }
    }
})(jQuery);

social_commentary_social_commentary.info.yml

name: Social Commentary
type: module
description: provides Facebook comments tab on enabled content
core: 8.x
package: HU8

social_commentary_social_commentary.libraries.yml

facebook_comment:
  js:
    js/facebook-comment.js: {preprocess: false}

social_commentary_social_commentary.links.task.yml

social_commentary.social_comment:
  title: 'Social'
  route_name: social_commentary.social_comment
  base_route: entity.node.canonical

social_commentary_social_commentary.module

<?php

/**
 * @file
 * Contains social_commentary.module..
 */

use Drupal\Core\Routing\RouteMatchInterface;

/**
 * Implements hook_help().
 */
function social_commentary_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    // Main module help for the social_commentary module.
    case 'help.page.social_commentary':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('provides Facebook comments tab on enabled content') . '</p>';
      return $output;

    default:
  }
}

social_commentary_social_commentary.routing.yml

# In order to to create pages it is necessary to define routes for them.
# A route maps a URL path to a controller. It defines what function
# or method will be called when a URL is accessed.
# If the user accesses http://drupal8.dev/node/{node}/facebook, the routing
# system will look for a route with that path. In this case it will find a
# match, and execute the _controller callback. In this case the callback is
# defined as a classname
# ("\Drupal\social_commentary\Controller\DefaultController")
# and a method ("content").
social_commentary.social_comment:
  path: 'node/{node}/social'
  defaults:
    _controller: '\Drupal\social_commentary\Controller\DefaultController::content'
    _title_callback: '\Drupal\social_commentary\Controller\DefaultController::title'
  requirements:
    _custom_access: '\Drupal\social_commentary\Controller\DefaultController::access'

social_commentary_src_Controller_DefaultController.php

<?php

namespace Drupal\social_commentary\Controller;

use Drupal\Core\Access\AccessResult;
use Drupal\Core\Routing\Access\AccessInterface;
use Drupal\Core\Controller\ControllerBase;
use Drupal\Core\Session\AccountInterface;
use Drupal\Core\Url;

/**
 * Class DefaultController.
 *
 * @package Drupal\social_commentary\Controller
 */
class DefaultController extends ControllerBase implements AccessInterface {

  /**
   * @return string
   *   Facebook comment tag.
   */
  public function content($node) {

    // Create a meta tag for the facebook app id.
    $app_id = array(
      '#tag' => 'meta',
      '#attributes' => array(
        'property' => 'fb:app_id',
        'content' => array('#plain_text' => '12345'),
      ),
    );

    // Return content, including tags and js libraries.
    $options = ['absolute' => TRUE];
    $url_object = Url::fromRoute('entity.node.canonical', ['node' => $node], $options);
    return [
      '#type' => 'markup',
      '#markup' => '<div class="fb-comments" data-width="100%" data-href="' . $url_object->toString() . '" data-numposts="5">Facebook comments for this content are loading...</div>',
      '#attached' => array(
        'library' =>  array(
          'social_commentary/facebook_comment'
        ),
        'html_head' => array(
          array(
            $app_id, 'app_id'
          ),
        ),
      ),
    ];
  }

  /**
   * A custom access check.
   *
   * @param \Drupal\Core\Session\AccountInterface $account
   *   Run access checks for this account.
   *
   * @return AccessResult if the access is allowed.
   */
  public function access(AccountInterface $account) {

    $node = $this->getNode();
    // Check permissions and combine that with any custom access checking needed. Pass forward
    // parameters from the route and/or request as needed.
    return AccessResult::allowedIf($account->hasPermission('access content') && $this->hasFacebookCommentEnabled($node));
  }

  public function title() {
    $node = $this->getNode();

    return t('Social comment on %title', array(
      '%title' => $node->title->value)
    );
  }

  /**
   * Determine if this node has a Facebook social comment enabled.
   *
   * @return boolean the Facebook comment status.
   */
  protected function hasFacebookCommentEnabled($node) {
    $facebook_enabled = ($node->hasField('field_facebook_comment')) ? $node->field_facebook_comment->value : FALSE;
    if ($facebook_enabled) {
      return TRUE;
    }
    return false;
  }

  /**
   * @return \Drupal\Core\Entity\EntityInterface|mixed|null
   */
  private function getNode() {
    $node = \Drupal::request()->attributes->get('node');
    if (!is_object($node)) {
      $node = \Drupal::entityTypeManager()
        ->getStorage('node')
        ->load($node);
    }
    return $node;
  }
}

Hardwired \Drupal:: is to be avoided (it makes the code harder to test or reuse), so I'd recommend replacing one with $this->entityTypeManager() which ControllerBase gives you, and the other with:

  protected $requestStack;

  /**
   * {@inheritDoc}
   */
  public static function create(ContainerInterface $container) {
    return new static($container->get('request_stack'));
  }
 
  public function __construct(\Symfony\Component\HttpFoundation\RequestStack $requestStack) {
    $this->requestStack = $requestStack;
  }

This turns your getNode() into:

  private function getNode() {
    $node = $requestStack->getCurrentRequest()->attributes->get('node');
    if (!is_object($node)) {
      $node = $this->entityTypeManager()
        ->getStorage('node')
        ->load($node);
    }
    return $node;
  }

The DI is a few extra lines but it saves you heartache in future. For example, if you want to decouple the entity type manager too, you only need to modify a couple of lines:

  protected $requestStack;

  /**
   * {@inheritDoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('request_stack'),
      $container->get('entity_type.manager')
    );
  }
 
  public function __construct(
    \Symfony\Component\HttpFoundation\RequestStack $requestStack,
    \Drupal\Core\Entity\EntityTypeManagerInterface $entityTypeManager
  ) {
    $this->requestStack = $requestStack;
    $this->entityTypeManager = $entityTypeManager;
  }

@jpstacey - I presume in getNode()

$node = $requestStack->getCurrentRequest()->attributes->get('node');

must be

$node = $this->requestStack->getCurrentRequest()->attributes->get('node');

Yes, sorry - copypasta error