It's a simple question: how many Freemasons are there in the world/US/wherever. The statistics can be hard to gather. You'll hear numbers like 6 million or 5 million, but these numbers don't give a clear picture. I can clear this up a bit...
As you can see, the graph looks rather stark. The decline in the US that we hear about and are often told began in the 1970s or 1980s actually began in either 1960 (total number) or 1955 ("per capita" or fraction of the total population of the US). Why are these different? As with most questions regarding the US population in the 20th century, the answer involves the Baby Boomers, WWII vets and the changing landscape of our culture in the 1960s.
But starting in the 1960s, even the WWII vets couldn't prop up the numbers any longer and the fact that the Baby Boomers were not joining in the same numbers as their parents became clear. Over the next 40 years, the decline was as steady as a heartbeat. It was only in the 2000s that the number began to show signs of leveling out, but of course, by this point the numbers were radically declined, and it's possible that there are just dynamics of a smaller population that take over.
But there are some interesting hints in the data. The existence of an existential threat that both unifies the population and encourages it to view its history with pride seems to have a significant effect on membership. Notice the drastic change in trajectory between 1940 and 1942. The numbers hadn't changed much, but the trajectory went from a major slide down to a sharp spike up!
But is membership all we care about? It's nice to have the money that comes from 4 million dues-paying members, but is it necessary? I don't think that it is. We're a link in the chain of the Western initiatic tradition. Ultimately we will contribute to the next link in that chain and they to the next and so on. What is critical is that we maintain a sense of why that chain is indissoluble for long enough to connect to whatever comes next; to pass that wisdom on to the Academy that will teach the next generation of seekers. Will that be in 5 years or 50 or 500? There's no way to know.