Yes, because while I can see it's fad-ish, it also serves a need. My friends use the event invites to schedule their parties and it's really easy, since everyone already has a facebook account.
If I tried to get them to switch to another party planning service, I know the answer will be 'It's too hard to get everyone to make an account there. We've already got accounts here.'
I have seen the RSVP scale slide from yes means yes to yes means maybe, maybe means no and no means hell no. Your group of friends may be different, but mine tends to either use email or do invites in person. From this point of view FB is useless to me.
I RSVP'ed to a demonstration against Norway's new data retention law, to take place in my town's major square. Several thousand people had RSVP'ed "yes". I was actually quite psyched at the tought of our own "Tahrir Square" moment. Ultimately, perhaps a total of 100 people showed up. The law passed.
If I tried to get them to switch to another party planning service, I know the answer will be 'It's too hard to get everyone to make an account there. We've already got accounts here.'