There's a few people keeping the practice alive, like Global Penfriends and etc. I don't know how many choose to use snail mail for it, but I know of people who for various reason enjoy physical correspondence. Probably most pen pals are using email these days -- although that's getting a little archaic now, too!
I have elderly relatives an ocean away from me and they absolutely refuse to text, so we talk via email. It's kind of like having a pen pal, sort of.