public interface IFacebookservice{
  InviteFriends (string appLinkURL, string previewImageURL);
}

public void InviteFriends (string appLinkURL, string previewImageURL)
{
            var fromController = UIApplication.SharedApplication.KeyWindow.RootViewController;
            var content = new AppInviteContent {
                AppLinkURL = new NSUrl (appLinkURL),
                PreviewImageURL = new NSUrl (previewImageURL)
            };
            AppInviteDialog.Show (fromController, content, null);
}

public void InviteFriends(string appLinkURL, string previewImageURL)
    {
        if (AppInviteDialog.CanShow())
        {
            var activity = Xamarin.Forms.Forms.Context as Activity;
           var content =new AppInviteContent.Builder().SetApplinkUrl(appLinkURL).SetPreviewImageUrl(previewImageURL).Build() as AppInviteContent;
            AppInviteDialog.Show(activity, content);
        }
    }

var facebookservice=Dependency.Get<IFacebookservice>()
    button.click+=delegate{facebookservice.InviteFriends("appurl","previewimageurl")}

AppInviteDialog AppInv = new AppInviteDialog(activity);
        AppInv.RegisterCallback(callbackManager, invitecallback);
        AppInv.Show(content);
        var invitecallback = new FacebookCallback<AppInviteDialog.Result>()
        {
            HandleSuccess = Result =>
            {
                if ((string)Result.Data == "Bundle[{didComplete=1}]")
                {
                    textbox.Text = "Friend Invited";
                }
            },
            HandleCancel = () =>
            {
                textbox.Text = "Cancelled";
            },
            HandleError = Error =>
            {
                textbox.Text = "Error" + Error.Message;
            }
        };

var dlgt = new MyAppInviteDialogDelegate();
    var dialog = AppInviteDialog.Show(fromController, content, dlgt);
    public class MyAppInviteDialogDelegate : AppInviteDialogDelegate
    {

        public override void DidComplete(AppInviteDialog appInviteDialog, NSDictionary results)
        {
            textbox.Text = "Friend Invited";
            if (results.Description.Contains("cancel"))
            {
                textbox.Text = "Cancelled";
            }
        }

        public override void DidFail(AppInviteDialog appInviteDialog, NSError error)
        {
            textbox.Text = "Error" + error.Description;
        }
    }