I was in high school in the 70's- graduated in '74. I raced in a club in Northern NJ, Nutley Bike Club. I did club races and did Somerville and the state championships. There was a high school bike club of about 5 (including a teacher). We did some club rides, but mostly I rode on my own. My first "serious" bike was a Peugeot UO-8. My best friend in HS and I road, but when I started racing I rode on my own (he didn't like racing).
Bike and cycling were not "elitist" so much as enthusiast driven, although I do remember high school kids giving me puzzled stares as I told them my Campagnolo cranks cost $120 (they thought I was insane). My biking friends and I followed racing- Tour de France and Merckx and John Howard and domestic racing. We subscribed to VeloNews- then a newspaper. I became infatuated with everything Italian- Campagnolo, Clement, Vittoria etc. I had a Raleigh "Competition" for a while, then worked in bike shops and saved and graduated to a Guerciotti full Campy Record (still miss that bike). These were the days of nail-on cleats and leather "hairnet" helmets. I remember shaving my legs for the first time and getting puzzled suspicious looks from my girlfriend in HS.
The so called "bike boom" was certainly a boom in my mind- I was obsessed with cycling and racing and European racing and Gran Fondos. The "boom" it seems to me, was driven by the growing import of better equipment and teens (like me) or 20 somethings in college (which I was by '75 in New York City) creating a growing and graduated market for imported upscale bikes. Stuyvesant Bicycle Shop in NYC was a mecca for cool bikes and components from Europe at the time. I used to ride my chrome Atala track bike around NYC. It was heaven when they decided to close Central Park on the weekends. NYC was progressive even back then- closing the park for cyclists, runners and skaters despite protest from taxis and drivers. I did the "Pepsi Challenge" and road a century in the park back then. Many hundreds were on that ride- I think around 1973 or so.
The gasoline shortage of the mid 70's definitely boosted bike sales around my area as more people turned to bikes as an alternative to driving- especially when they sold gas alternating on days according to odd/even plate numbers. I was just starting to drive, and i turned to my bike as a much more reliable means of transport. My bikes became utilitarian as much as for racing and sport riding.
Another bit of a phenom was the growth of the American Youth Hostels (AYH) that held large teen bike tours through New England and all around the country with a network of hostels for extended overnight trips. I never managed to go on one, but I had many high school friends that participated and i am sure they contributed to a generation of cycling enthusiasts.
I would guess that the cycling "boom" was mostly located on the east and west coasts because the growing distribution of new bikes and components branched out from importer located near major shipping distribution points- NYC, MD, LA, SF, etc. As others have reported, my sense and memory of the boom was it was driven by teens and college students and more as a utilitarian alternative to cars for campus and in urban locales. As excellent as NYC mass transit was for convenience (although very dangerous back in the 70's unlike today), I rarely had to use the subway- rode my bike. I miss that all chrome, cottered crank, sew up Atala track bike- still dream about riding it.