Most people don't RSVP and it's incredibly rude.
That being said, we did the facebook invite for DD's birthday party and all but 1 person RSVP'd, so that was impressive. The person who didn't RSVP was considered a 'no' and they did not show up.
SAHM to Magnolia May (09/10), Luke Russett (04/13) and expecting 11/16. Wife and best friend to my airman.