Heather, you are definitely not alone. Our wedding is in December and we finally received two RSVP "Yes" few days ago. Since we are not going the all-inclusive route, our guests are free to stay anywhere they want and there is no time constraints in terms of booking. We have set September 18th as the RSVP deadline and hopefully we will be getting more replies.
Glad we didn't bother doing RSVP through mail. Otherwise it will be a total waste of money and effort. I don't understand why it is so difficult for guests to RSVP online!