Invitations sent but no one RSVPing yet!

I have sent out all my invites over last couple weeks and only got 17 people that rsvp'd. I know people are busy but I'm afraid some are not reading the rsvp card right. Cause we had couple already ask us how but says on the card to go to the link and rsvp online. These people are fairly young too. So thinking maybe sending Facebook message or status saying" invitations are all sent remember to rsvp by may 20th latest thank you ! " since I don't have everyone number and be quicker to do a mass message. Or would that be not good thing to do right now ?