We had two. Both evening guests so it didn't matter catering wise but it was still a massive kick in the teeth.
One couple said that they couldn't get a babysitter and weren't able to come (via facebook messages). Bullpoo. They rsvp'd about 8 weeks before so had plenty of time to organise something. Fine. Got over it. Now I've just seen some photos dated 6.7.13 "Barbecue with the Fam". I feel wounded! We were mugged off for a barbecue - they haven't even sent a card!
The other was a friend who sent a text about an hour before the wedding saying that she had a family emergency. Again. Bullpoo. Haven't heard anything about this family emergency since and she then went on holiday the following day. Again, no card.
Call me old fashioned, but a polite congratulations/sorry we couldn't make it card would have been nice!