When Stan Lee spoke at Emerald City Comic-Con he dropped an interesting tidbit.
As a kid, he was a fan of Leo Edwards' now-forgotten Jerry Todd books. They were adventure stories "like the Hardy Boys, but with humor."
That's not a bad description of Stan's style of superhero comics. But a specific feature of the books had another, more particular influence: they had letters pages, with replies by the author. Hardbound books!
So that's the inspiration for the Marvel letters pages.
But Stan did something new: he printed full addresses. This allowed fans to write each other.
Most famously, Richard Pini contacted and wooed Wendy Fletcher after she had a letter printed in Silver Surfer. They married and gave birth to a bunch of elves.
But I think thousands of connections were made, fanzines concocted, adzines circulated, because of those pre-internet, printed addresses.
Here's a Leo Edwards tribute site. Everything has a page on the internet!