Ladies, also keep in mind that things do get lost in the mail! So a call or email may be in order. Just don't do it until after your RSVP date has passed. An example of this happening is when I spoke to my best friend from college on the phone and she actually sent in the RSVP card via snail mail and I never received it. She had booked and we speak regularly so I already knew she was coming, but there are plenty of others I do not speak to regularly like my parents' friends and stuff. I have had invitations not make it to people, etc. It happens more often than you think. So don't assume that just because you haven't heard from them that they are being rude and not RSVPing.
I have an option to RSVP via my website as well but it seems most folks really like doing it the old fashioned way via snail mail or by calling and telling me or my parents. I do not care how we get notified, just as long as we do get notified.