Email/newsletters are a promising protocol, but you would have to rethink the UX to be more like a newsfeed, where a concise version of the message is shown once, and then automatically marked as read.
I have noticed friends reverting back to RSVP e-mails via Paperless Post Flyer. I'm still not sure if one person in this social group is not on Facebook, causing the change, but it has been a welcome adjustment. If Facebook Events can be replaced by an e-mail approach, I don't see why other products can't follow suit.
