I used to be a letter-writer. I regularly exchanged long letters with a number of acquaintances. Now, with e-mail, that's all changed. The long letters have been relpaced with more frequent, postcard-length e-mails. And I lost touch with people who had no e-mail. So at least in my own experience, I agree, e-mail has had a very dramatic effect on personal letters as well as personal correspondence.
I think in part it did, but it found new functions. People no longer gather around the radio to tune in to the weekly "Lone Ranger" or trivia game shows. Radio provides different content these days. Radio is still useful as entertainment when we'd rather keep our eyes free (while driving, while sitting on the porch).
When I was 8, my neighbor told me that page 1 of the newspaper tomorrow was going to be edible, as an experiemental way of fighting world hungar. The next day, I told him it didn't taste very good. He asked me which of the two daily papers I ate. "Ah," he said. "That's your problem. I meant the other paper."
I get that by e-mail, as grelmar suggested.