I recall posting a comment on Facebook, a couple of years ago, saying that I was a bit upset that people no longer take the time to send a note of thanks when they've received a present/card from you. Although I didn't name and shame, the particular niece to whom I'd never received a response from, made quite a nasty comment about it and I then received a begrudging notelet giving 'thanks' in the post. We now have a family agreement that presents are only for the kids not the adults and we just send out birthday cards, her brothers likewise never bother to thank me for their cards but still, if you've received a card in the post shouldn't you be grateful that your birthday has been remembered?
The fact that they don't live nearby, and we rarely see them, it's not as if I can ask if they received it or, more appropriately, that they would have the opportunity to thank me in person.
Frankly, if I've taken the time to go and choose a card, sit down and write it, and then put it in the post, a quick thank you wouldn't go amiss? I mean, I don't even know if they've received it unless they respond to me, do I? They all know my email address and that I'm on Facebook so I'm not even expecting them to sit down and write a thank you note - all it takes is an email or a FB message which is certainly no hardship in this day and age.