There was one time when I put considerable effort into a Christmas card (when we did the secret santa exchange here) and never received so much as a 'thanks', so I wasn't even sure it was received. Then again, I received some amazing cards from people that time and didn't have the capacity to give back more than a 'thanks', so I felt a bit bad about that.
I'm fine with delays. I sometimes can take months to respond. Messaging/emails/texts can be a bit demanding sometimes and it's nice to know you can switch off when you need to. I've got some friends from whom I hear every 4 months or so, and that's fine. I always respond in the end, though. When somebody doesn't respond at all, it's a bit sad. I try to understand that people have busy lives and a lot of them prefer face-to-face interactions (why oh why?).
Leonard Cohen (Bird on a Wire): I have tried in my own way to be free
Mrs Hudson (BBC Sherlock): Sherlock! The mess you've made!