According to your figures during the 1948-1953 span, when yearly circulation was edging up to one billion issues, the U.S. population was about 160 million. That means the yearly per-capita purchase of comics then was about 6 comics for every man, woman and child in the U.S.To put that in perspective, if the same amount of comics were sold today, relative to the current population, that would be about 1,867,708,734 comics per year. Nearly two trillion. Compare that against what John Jackson Miller calculated to be about 69,200,000 in 2010. If you're not that great at math, the number of comics sold in 2010 was a little less than 4% of what was sold in 1950 on a per-capita basis. Instead of 6 comics per person per year, we're closer to less than a quarter of a comic per person per year. Keep in mind, too, that comic books in 1950 were about twice as long as they are now.
We know that every man, woman and child didn't buy comics, so it means that those who did, bought a lot more than the per-capita average. When you consider the pass-along readership was estimated at 5 readers for every copy sold, there's a very interesting conclusion to be drawn. During this era, comics were a mainstream entertainment medium for children and some older people...
With the huge circulation figures of the sort we see for the post-war/pre-code era, comics were definitely mainstream. When I was growing up, I struggled to figure out what motivated people to stir up the Comics Code fuss. Why were parents, Congressmen, etc. dedicating so much energy to cleaning up the contemporary comics? After all, I though, comics simply aren't that important. It took a while for me to realize there was a very different situation in that era -- comics really were that important back then. Television hadn't yet taken root and the comic book was a staple of a kid's entertainment, and that comics really must have been everywhere.
Now, that doesn't take into account graphic novels and bound collections. It doesn't take into account anything outside the Diamond distribution system (print-on-demand, digital, etc.). The Diamond reporting isn't terribly accurate, since we don't have actual numbers to work from. And, to one of the points Klein alluded to, we now have a wealth more options for our entertainment dollars.
But even with those caveats, it's still pretty sobering.