The days of legitimate RSVP, I'm afraid, are a thing of the past. The exception may be for weddings or VIP parties – where “by invitation only” is your ticket through guarded doors or a seat at an assigned table.

In the case of digital media, I couldn't agree more with your sentiment: “…when your RSVP response is public, you’re transmitting more than your actual attendance — you’re showing off your allegiance to the group, and also endorsing the event with your alleged attendance. (Maybe we need a new category to help hosts in their planning: “I would if I could, but I can’t, but you should.“)…”

I admit, I RSVP “yes” when I'm 60% likely to attend and “maybe” if I want to think about it, explore other options, or track the event. But mostly, if I take the time to choose “maybe” instead of “no”, it is because I want to promote or share it with others. And I think that's the point savvy marketers should pay attention to.

Back to my days of corporate event planning, analysis of the RSVP lists told us that there were invitees who consistently replied “yes” but never attended a single event. Upon sending a survey to these people, we learned several things.

(1) they RSVP'd “yes” so that they would continue to receive future invitations
(2) they had good intentions, but terrible follow through – ie not 100% committed
(3) they RSVP'd “yes” because they liked the idea and were forwarding to their colleagues/friends. But if they couldn't find a buddy to attend with, they would no show.

As a result of sending this survey, we saw our actual attendance more accurately reflect the RSVP list. Lesson learned = communication is key in understanding your audience. If you're an event planner, make sure you've got the right people on the list. The more targeted or relevant your list is, the better your RSVP results and ultimately the better your attendance.