> After about ten years on the market, Valium had been prescribed to 59.3 million patients 
The US population in 1970 was about 200 million. A significant portion of the adult US population took valium at some point during the 60s/70s.
Ridiculously high because they were touted as safer alternatives to the then popular barbiturates. Benzos are safer, it just turns out that benzos also have not-so-great long-term effects.