Oh wait.. even worse! Imagine if everyone worked basically the same hours - then people could predict when your house is likely to be empty and rob them!!
Oh wait.. we already have that. Better panic already I guess.
Second, everyone knows their Twitter/FourSquare posts are going to be public. Not so with FB.
Third, Facebook is the one with the established reputation of making public by default user information that many assume to be private.
I think the point is less about telling people that you're away and more about publicly broadcasting the location of other people's private information.
I can assure you I'm not a privacy nut, but I think we all have the right to keep the latitude and longitude of our homes hidden from 500 million other people if we want.
This doesn't make Facebook, Foursquare et al evil. People should know better. However, common sense should tell these companies it's probably best to keep home addresses private to avoid law suits/scandals. It just seems like the right thing to do.