Ok, so I'm a FB addict and I even think that is a little crass.
I definately would never RSVP in FB unless the invite was sent that way, then obviously that makes sense (I'm talking showers, not weddings! OMG could you imagine an FB wedding invite?! LMAO)
When Chris and I move we've already decided we aren't having a home phone, we are never home anyway, so it just doesn't make sense. Plus if people do call the house we never answer it anyway! HA! Voice mail baby, that's where it's at!
I remember the days before call display when I was a kid, no one ever answered the phone at dinner, it was not allowed!!