For what it's worth (not much) I think FB and yahoo are fine ALTERNATIVE methods of communication. I agree with Jon that RL can sometimes be a real bitch, so having something else as a backup reminder that people can opt in (or opt out) of, is probably a good thing.
Try setting something up, make it clear it is only a backup form of notification, and let people choose to opt in, and allow them to opt out. No harm, no foul. Just my 2 cents.
-Greg