Codice da inserire dopo "<body"
<div id='fb-root'/>
<script>
  window.fbAsyncInit = function() {
    FB.init({
      appId : 'APP_ID',
      xfbml : true,
      version : 'v2.7'
    });
  };
  (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_US/sdk.js";
     fjs.parentNode.insertBefore(js, fjs);
   }(document, 'script', 'facebook-jssdk'));
</script>



sostituendo:

APP_ID con l'ID della vostra app