You probably cannot provide a notification to let a user know that a web page has been modified, especially Facebook. To do that you would have to continually monitor the content of a web page , possibly using a Clock and a Web component) and send the 'notification' whenever there is a change, however, what you probably will get is a 'notification' sent every time an ad on that page is checked. You do know that App Inventor native Blocks cannot send notifications except locally on a device when your app is running. If the app is not running, (remember App Inventor 2 apps cannot run in the background) then what you want to do is not possible.