This.
Peoples memories are fuzzy. If you tell them a rumor started in a certain year, they’ll falsely remember hearing it in that year because most people want to part of the in-crowd. It’s just human nature.
Town crier.
Mimeographed chainmail was a thing in the 80s.
… which morphed into faxes in the '90s
Some suffering academic has surely studied this evolution.
There’s some irony here that rumors, lies and urban myths spread easily before the internet, but the internet has made the truth easy to find.
Example:
I have a friend who, a couple of decades ago, insisted that Charles Manson auditioned for the Monkees. When I asked her how she knew, it amounted to someone had told her and he’d told her it was true. So she believed him. When the internet came along, I was a bit of an early adopter, and developed an early fascination in rumours, myths and how they spread. I became pretty unpopular with people warning me by email about the Bud Frogs screensaver, sending me the Nieman Marcus cookie recipe etc.
I sent my friend the cites that showed that Charles Manson was not only too old, but was in prison at the time of the Monkees auditions. So without the internet, the rumor spread - but it should have been stopped dead as technology made it so easy to prove the truth.
Funny how it hasn’t worked out like that.
Karl Rove used state universities to spread false conspiracy theories about his political opponents. When the school’s took their holiday breaks, the students returned to their homes to effectively scatter the stories all over the state:
Some of Kennedy’s campaign commercials touted his volunteer work, including one that showed him holding hands with children. “We were trying to counter the positives from that ad,” a former Rove staffer told me, explaining that some within the See camp initiated a whisper campaign that Kennedy was a pedophile. “It was our standard practice to use the University of Alabama Law School to disseminate whisper-campaign information,” the staffer went on. “That was a major device we used for the transmission of this stuff. The students at the law school are from all over the state, and that’s one of the ways that Karl got the information out—he knew the law students would take it back to their home towns and it would get out.” This would create the impression that the lie was in fact common knowledge across the state.
Here’s one rumor:
And certain professions. A buddy has worked for a small city works department since the 80s. Back before the Intertubes, he used to come home with a new joke every day or three. A few weeks later, I’d start hearing it from other sources. I expect there were similar networks among other professions who often communicated across geographic lines–lawyers, for example.
(There was sort of a meme in our crowd before the word existed: my buddy can’t tell a joke to save his life. So he’d get home from work, come over, fumble his way through the joke, and I’d figure out how it SHOULD be told. Then I’d retell it, prefacing it with “This is [buddy’s] joke”, so he’d get the credit.)
Silly boy you.
It was a Zoom call. I thought everybody knew that 
Here’s how it worked
Musical Number: Bye Bye Birdie - Telephone Hour - YouTube
I wonder about the salacious ones that wouldn’t have been acceptable for TV or radio. I remember hearing the Rod Stewart stomach pumping story in Junior High (so somewhere between Sept. 1977 and June 1979), coming home from school and my sister, who went to high school in a different city, had already heard it that same day. Details were off (3 ounces vs 8 ounces) but the gist of the story was the same. Obviously it wouldn’t have been on TV or radio in the late 70s. It was all word of mouth. But where did it come from (no pun intended) originally?
I know rumors get recycled, as I remember hearing the story again in the 90s, only that time it was about Alanis Morissette.
^ It was Elton John when I was in Jr. High (circa 1974).
Rod Stewart says it was spread by a tour publicist who he fired for “entertaining” local men in his hotel room. I’m not sure which press he passed it to. Creem, maybe.
I see what you did there 
I don’t know if he coined the term but Brunvand is his books on urban legends referred to a whole subcategory as “Faxlore” which was a vector particularly for workplace based rumors.
And it was Marc Almond in 1981.
There were also conspiracy theory cassette tapes. In 1989 or so, one of my friends had one that laid out the “Paul is dead” CT. (For the record, neither of us believed it)
But a lot of stuff, especially if it was easily digestible, was spread through simple word of mouth.