My mom works at the post office and she says that a lot of wedding invitations do get torn up in the mail. The reason being is with the invite, the reponse card, and whatever else jammed into one envelope, they normally get ripped open on the machines. If that is the case, they try to tape/fix them and mail them or try to return to sender. You could try calling the p.o. you sent them from and see if they have any of your invite that got torn up.

Odds are though, it's probably just that your guests haven't replied becuase they have overlooked the date. I would give it a few more weeks then if you aren't getting any at all, I would definately start calling people.