Tell me, what's easier:
a) Create a new event -> Invite everyone on friends list or everyone who's likely to be interested -> whoever wants to go RSVP's -> Fun
b) Individually call/text 25+ people to let them know something's about to go down
In practice, both are necessary. Facebook's useful to gauge initial interest in an event, but I find it almost necessary to chase down people via text or IM to get their real RSVPs (confirmed = "most likely will go but I might drop out at the last second," maybe = "nah, not really interested but might come if I'm bored," invited = "I don't know you, or definitely not going", and declined for people that **** you off )
Either way, I do notice a marked decline in activity on Facebook but I don't plan to abandon it anytime soon, even though it's generally littered with copies of my tweets. I do like how we've moved on from the crazy days of people's walls filled with two dozen apps - and app invite requests can be easily ignored.
... and finally, this is relevant to this thread