I have a list of ~10 people I would consider "close", immediate family and good friends, and 5 or 6 more tertiary contacts. I travel fairly frequently, so I had plenty of opportunities for sending postcards. I send cards for obscure holidays just because. The physical process of hand-writing messages is so therapeutic for me. I've probably sent ~250 postcards in the last year and a half.
I have received... 3 physical responses. It has been extremely disappointing, but I continue to send mail because I enjoy the process of writing the cards, and the knowledge that people probably appreciate the mail makes me feel good, so at least I get a little out of it myself.
My mom will occasionally text to say she liked the postcard, but has never bothered to send one back to me.
I would be delighted if more people chose to communicate slowly.