This assumes the recipients have consistent addresses, and requires significant additional time and labour. Why would he drop Facebook in favour of snail mail, when e-mails (a more convenient version of the same thing) provided an inadequate solution for him in the past?

You can put just as much time and thought into an email as you can a letter. Technology is not the issue here, merely etiquette.

I wrote about the difference between snail mail and letters here: https://news.ycombinator.com/item?id=15031357