SF_10.1, SF_10.2 - https://docs.sitefinity.com/for-developers-use-the-ipageprerendercompleteevent

FacebookMeta_Global.asax.cs

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.Initialized += Bootstrapper_Initialized;
        }

        void Bootstrapper_Initialized(object sender, Telerik.Sitefinity.Data.ExecutedEventArgs e)
        {
            if (e.CommandName == "Bootstrapped")
            {
                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));
                    }
                });
            }
        }
    }
}