We know, looking at the initial list of performers et al in the last entry, that fanzines are a recent invention. So how did fans used to communicate? Was there even such a thing as a fan? Before the collector market gathered momentum and magazines sprang up to cater for the kind of reader who kept each issue and collected other related titles, there was the letters page, now largely defunct.
I would question whether this was the ideal vehicle for the fan. It promoted camraderie, brought creator and reader into close commune,...
but the letter writer was restricted in how far he or she could deviate from commenting specifically on last issue's storyline. This is fine if you're a Viking Prince fan and his series is running in Brave and the Bold but what if you're a fan of Quicksilver and it's not the nineteen forties? Too late.
Marvel's version may never have been as popular as the scarlet speedster (take your pick) but he's an intrinsic part of the Marvelverse: son of Magneto, stepson to the Whizzer and Miss America, brother of the Scarlet Witch, husband of Crystal. I'm sure he has his fans. Those fans at their earliest, could not wax too enthusiastically about one of the bad guys in X-Men, anymore than they could draw all the focus away from the other stand-ins who called themselves Avengers. The odd mini-series aside, he has been all but cast to the dark side of the moon, an Inhuman fate.
The kind of people who write in to comic book companies learned to stick to the point. Though that didn't mean they wouldn't slam the creators, note slips in continuity, praise and damn with equal gusto. It was fun to see what readers were thinking and you didn't have to join some club or sign on to some forum to access it. The promise of being immortalised in your favourite book or series has to be among the ultimate prizes in fandom. Or no-prizes even.
Companies varied in their perceived demographic and the letters pages reflected these differences. With some letters page editors, you didn't need to be worldly wise to comment, just be able to tell Gary Friedrich from Mike Friedrich and recognize a Herb Trimpe Foolkiller from fifty paces.
As for letter hacks, they were the elite. I don't recall the exact terminology but I think you were a Fearless Face Fronter if you'd a letter published in both Sock It To Shellhead and Let's Rap With Cap. There were letter hacks notorious in their day as jgoldman10 is in the digital age. Actually TM Maple and Commander Quotey [and his lost marvel madmen - after the arguably more obscure Commander Cody and his lost planet airmen] were required to be more thoughtful and keep their comments relevant, however flippant their guise. Some letter hacks even went from writing letters to writing the book itself: Bill Mantlo and Roy Thomas began that way.
I'm nothing if not thorough so we will look sidereally at 'letters to the editor' in newspapers and magazines or, rather, we'll hyperlink to a couple of examples and you'll get the general drift.
Journalism can't help but touch on our customs and our built environment, whereas comics letters pages do so only on a whim and if there's a way of linking the critique/discussion to last issue's story or artwork. Or (as is the case in multi-story British comic books ) the brand at large.