Yup, that's what I was thinking too. I'm nearly done working on something just for that, will post to HN soon
I built a Facebook app that does this for personal use. It's broken now since they released API changes, but it's pretty easy to do. Maybe I should go back and fix it and open source it.
Not sure what Facebook's policies say about these sort of notifications though.
Re:Show HN: Breakup Notifier
by nickpatrick on Sunday March 06, 2011 @03:28PM
(#2294941)
Just launched my app for this. http://www.defriendalert.com
This is actually what I thought you had built, and for me would be even more useful + interesting.
(Or, open source the code and let someone else do it)