You pick the friends you want to stalk, and the app will send you an email every time they change relationship status. The idea is that you'll know when a crush becomes available so you can then make your move. If you're in a relationship when you sign up, the site will gently reprimand you, saying: "P.S. You're in a relationship! Naughty naughty . . ."
But Breakup Notifier serves purposes beyond helping you score a date. With the tool, the dedicated stalker can streamline the process for finding out when a Facebook friend goes from in a relationship to engaged, engaged to married, and, as more time goes on perhaps, from married to single. More reason to hide your status!