Yes, exactly. You can sign up to Facebook, but you may not get the notifications you were expecting. You can even set up notifications via email or text message so that you don't have to log in to the site, but what you will get is a stream of of crap as a Facebook AI's feeble attempt to draw you back in to Facebook. But the notifications that you might actually want? Not so much. And even if you do they will be drowned in spam.
Also, in order to have got reunion messages from his school, he'd have to have 'liked' their page. It's kind of a dopey way to advertise a reunion really.