I think its more "if the party is of a certain size you don't notice who isn't there." so for acquaintances, dropping off FB can drop you out of a lot of invites.

That said, I find it challenging to remember my immediate families birthdays (not to mention, anniversaries etc) and while my crutch has always been a calendar, FB can provide that crutch for folks who don't have a calendar habit.

Bottom line is that the core premise of FB is actually quite sound, its just not easily monetizable. Its possible that after a while folks would pay to subscribe to a FB like service but long before that Google or someone else who isn't being held against the wall with a share certificate pointed at their head may find a more workable solution.

And of course FB could, their are a lot of smart folks over there.