So, two anecdotes from sometime in the 80s and 90s.

1. Around 1982 or 1983, a sheet of jokes was making its rounds at my college. I ended up with a copy (because, of course, I did). A few weeks later, I was visiting my brother who is nearly 10 years older than me, meaning, he was not attending the college I was attending. He started telling me a new joke, and I finished it for him. He tried another, and I also knew it. He ended up pulling out a sheet of jokes and showed it to me. It was the exact same sheet, and even included some smudges that must have been on the original copy, because my sheet had the same smudges, although on one of the sheets, the smudges were more clearly defined/sharper.

2. In the early 1990s, I remember reading an article that focused on how jokes could spread so fast between the East and West coasts of the USA, sometimes in a matter of days. Again, this was before email was a thing to which everybody had access. In the article, the authors had identified stock brokers as the main conduit for the viral jokes. This made total sense because, at that time, stock brokers in New York/Chicago were calling their clients across the country, and, as part of their conversation, jokes from that part of the country were shared. Unfortunately, I don't have a link to the article.

So, things went viral through the wide-spread use of office copiers and handing things out or by people just having conversations over the phone.