I did a spot of penpalling while I was in high school and university, i.e. before the Internet came to be in commonplace public use. It was of course common to make penpals through penpal organisations which worked in school, school exchanges/trips and holidays. Through all of these channels I've had penpals. In fact, even after I got online one or two e-mail correspondences moved to snail mail.
Unfortunately I could see that time and time again, for people to have a penpal correspondence with me did not work out. I attribute this to my odd behaviour which would have come across as odder in the past before I tried to "modify" myself, and restricted interests. I'm sure this would come across even when I was writing letters in a foreign language. And I'm also sure people would get some sort of snapshot of my life from my letters and feel that things were not quite the norm.
I make allowances that maybe letters get lost in the post occasionally, but surely not with most penpals. Surely most of the time they simply stopped corresponding. For example after I'd been on one trip abroad to a school we were twinned with, where pupils from both schools were assigned to each other and you stayed at their home, penpal correspondences naturally occurred. However when I wrote a letter to this pupil, instead of getting a reply from him I got one from his mother, to explain that he didn't wish to correspond. And on a return visit to our school, with the same pairs assigned, he (alone out of his class) did not come over.