[eloeffler]

It makes sense that it flourished when sending letters was cheap, plus it was a very common thing for adults to send letters, so why not add one for fun.

But I don't think a high cost of sending a letter is much of a hindrance if you pay it for social contacts. If you look at having a pen pal as a socializing event, then it's hard to beat how much it costs to go to a bar, concert or sports game etc.