Use the IPagePreRenderCompleteEvent

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web.UI.HtmlControls;
using Telerik.Sitefinity.Abstractions;
using Telerik.Sitefinity.Model;
using Telerik.Sitefinity.Services;
using Telerik.Sitefinity.Web.Events;

namespace SitefinityWebApp
{
   public class Global : System.Web.HttpApplication
   {

       public class FacebookMeta : HtmlMeta
       {
           public FacebookMeta(string propName, string content)
               : base()
           {
               this.Attributes.Add("property", propName);
               this.Content = content;
           }
       }

       protected void Application_Start(object sender, EventArgs e)
       {
           Bootstrapper.Bootstrapped += Bootstrapper_Bootstrapped;
       }

       private void Bootstrapper_Bootstrapped(object sender, EventArgs e)
       {

           EventHub.Subscribe<IPagePreRenderCompleteEvent>((x) =>
           {
               if (!x.PageSiteNode.IsBackend)
               {
                   var page = x.Page;
                   var siteNode = x.PageSiteNode;

                       //add custom class to body tag
                       var customClass = siteNode.GetCustomFieldValue("BodyClass");
                   var script = @"$(""body"").addClass(""{0}"");".Arrange(customClass);
                   page.ClientScript.RegisterStartupScript(typeof(Global), "custom", script, true);

                       //facebook meta graph
                       page.Header.Controls.Add(new FacebookMeta("og:title", siteNode.Title));
                   page.Header.Controls.Add(new FacebookMeta("og:site_name", "yourite.com"));
                   page.Header.Controls.Add(new FacebookMeta("og:description", siteNode.Description));
                   var images = siteNode.GetCustomFieldValue("Image") as IList<IDataItem>;
                   var image = images.FirstOrDefault() as Telerik.Sitefinity.Libraries.Model.Image;
                   if (image == null)
                       page.Header.Controls.Add(new FacebookMeta("og:image", ""));
                   else
                       page.Header.Controls.Add(new FacebookMeta("og:image", image.Url));
               }
           });
       }
   }
}