Facebook app can notify you when your friends change their relationship statuses
More: Facebook app lets you stalk -- er, monitor relationships - CNN.com
It lets users pick certain friends whose relationship status they'd like to monitor. If one of those relationships changes, the user gets notified by e-mail.
"You like someone. They're in a relationship. Be the first to know when they're out of it," promises the app's website.
So, if that old high school flame suddenly becomes "single" or the receptionist you've pined for decides that her on-again, off-again relationship is now "complicated," you can swoop right in.