I think that people don't know what to do with RSVPs but I think that doesn't mean to give up on them. I have always followed up after the deadline if I've heard NOTHING from someone and I need a headcount.
I think a lot of people think RSVP means to call only if you are coming and it doesn't. It means respond, please. So either way, you should RESPOND to the invite.
My uncle never responds and his wife is flakey. It was too hard for a grown man to check a box and put a card in a pre-stamped, pre-addressed envelope for my wedding. But my sister (at the time not really speaking to me) had the smarts to do it. Some people....
I also hate it when people say they'll come and you wait and wait and nothing. Kids with no one at their parties. People waiting to start a meal when the person changed their mind. How hard is it to call or email? Really. If my uncle's wife can post on FaceBook 50 times a day, she can send a note saying she won't be at the baby shower.
Edit: even Evites need to be followed up on but you can send a blanket message to all invitees. Just don't wait to follow up. Friend found out that the daycare person who was supposed to send out the email to the kids in her son's class (privacy issues) messed up the mother's email address and forgot the phone number so no one COULD RSVP! Only 2 kids came, and they were friends notified through FaceBook.