I think the bike boom was more of a bike-buying boom, than a bike-riding boom.
We rode bikes, because we were kids; and they were our transportation and entertainment. I always loved being on my bike- and spent a lot of time riding it.
c. 1975 an older relative got a geared Sears bike- that was the first case I had heard of, of an adult getting a bicycle. He rode it two times, than it sat for a few months, until he gave it to 13 year-old me! I heard of one other adult getting a bike- a relative of a neighbor; but it was reported that he had a "lot of problems with it" because it was one of them "new-fangled bikes with so many gears".
I progressed to a "10-speed" shortly after that. Kids rode bikes. it was the norm; you'd see it everywhere- but I don't recall ever seeing an adult riding a bike, except for one guy, who was a little ********, and went around doing odd-jobs, using his bike for transportation.
Bicycle racing? Club rides? Never heard of such things.
I had never even set eyes on a "good bike" back then. It was Schwinns; Huffy's; Ross's; Free-Spirits; Royce-Unions.....
Only wish I had stayed with cycling, or at least, would have taken it up again, sooner- 'cause I wasted 3 decades not riding!
[Ooooo-K, so RE- TAR DED gets cernsored areound here?!]