title: Send-to-messenger plugin not showing
body: <p>My Send-to-messenger plugin is not appearing on my website. 
  Here is my config:</p>
  
  <pre><code>&lt;script&gt;
  window.fbAsyncInit = function() {
  FB.init({
    appId            : 'xxxxx',
    autoLogAppEvents : true,
    xfbml            : true,
    version          : 'v3.0'
  });
  };
  
  (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 = "https://connect.facebook.net/en_US/sdk.js";
   fjs.parentNode.insertBefore(js, fjs);
   }(document, 'script', 'facebook-jssdk'));
  &lt;/script&gt;
  
  &lt;div class="fb-send-to-messenger"
    messenger_app_id="xxxxx"
    page_id="vvvvv"
    color="blue"
    size="large"&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>The html generated by this code on this page: <a href="https://www.weekendr.eu/bonsplans" rel="nofollow noreferrer">https://www.weekendr.eu/bonsplans</a> seems ok, but the button would still not appear.</p>
  
  <p>Some elements which may help :</p>
  
  <ul>
  <li>I'm on prod env</li>
  <li>My website has https: <a href="https://www.weekendr.eu" rel="nofollow noreferrer">https://www.weekendr.eu</a></li>
  <li>My app is published and online</li>
  <li>I'm admin and connected to FB besides</li>
  <li>I allow third-party cookies</li>
  </ul>
  
  <p>Thanks !</p>