There had been fan clubs before. The Merry Marvel Marching Society shamelessly stole its name from the Mary Marvel Marching Society.
Don Markstein's Toonopedia says the same thing:
She even had her own fan club, the Mary Marvel Marching Society (the name of which Marvel Comics shamelessly appropriated in the '60s, as the Merry Marvel Marching Society).
Except it's not true. There never was a "Mary Marvel Marching Society", and the name of the "Merry Marvel Marching Society" wasn't taken from an old Mary Marvel fan club.
The whole thing is a hoax. Larry Ivie (a comics fan turned pro who had a small career in comics during the 1960's) made up this story, and even went as far as making up a fake ad for the "Mary Marvel Marching Society" in order to support his claim (Ivie published his "evidence" in his magazine Monsters and Heroes). For some time, his story was believed to be true, but further research by Fawcett comics experts has shown that there was a Mary Marvel club, but no "Marching Society".
Some years ago, the following messages from P. C. Hamerlinck and John Pierce were forwarded to the Timely-Atlas mailing list.
P. C. Hamerlinck:
There was a Mary Marvel club, not a Marching Society. I'm 99.9% sure the
'Marching' ad was bogus. I'll forward your email to John Pierce who might be
able to shed more light on the subject, as we may have discussed it at one time
Although I once wrote an article and cited Larry's "evidence," apparently
it was false. Larry [Ivie] apparently had a grudge against Marvel, and wanted
to make them look bad. Or so I have been told.
Ivie's hoax has proven to be widely succesful unfortunately, and as can be seen above, is still widely believed by many people.