During the period from 1955 to 2000, membership in Freemasonry as a percentage of population dropped with what I find to be shocking linearity (about 1% per capita every 10 years, source data at http://www.msana.com/msastats.asp).
What I want to know is, was this extremely flat curve from 1955 to 2000 replicated in the various other fraternal, social and religious societies? Is there any kind of concrete data for this? I'm not talking about churches, but rather groups like the Odd Fellows, Knights of Columbus, the various social clubs (at least the ones that are large enough to have any public data), etc. What I've been told is that the decline in such public clubs and societies was relatively uniform, but I have no data to back that up.
Thanks!
PS: If anyone has an explanation for the very linear drop in numbers, I'd also be curious about that. My gut feeling is that, without the surge of membership influx created by a public perception that such organizations were how one learned to become a member of society, the numbers simply declined through attrition to the modern levels that represent, essentially, the background interest in joining such groups.
[Mason here]
I don't have data, but can point you where to get some.
The Masons are a special case, as far as gathering data is concerned: you're lucky to have found a compilation.
Freemasonry is very, very decentralized. In the US, there is generally speaking, one 'mainstream' Grand Lodge and (at least) one 'Prince Hall Affiliation' GL per state; not to mention many other bodies with varying claims of legitimacy. To get complete historical numbers would require querying each one individually. I suspect the MSANA numbers are just the 'mainstream' GLs. All or almost all GLs have websites with contact info; you would go through the Grand Secretary's Office to find the right people to talk to.
Other orders (Oddfellows, KoC, etc) are easier, since they have central organizations which may be able to provide you with data.
It would be illuminating to look at the per-year numbers of initiations, demits and expulsions (members leaving, or being dropped (mostly for non-payment of dues)), and deaths.
As for the reasons for the decline, I'm not aware of any serious academic studies. The topic is endlessly debated, and I suggest you ask over on /r/freemasonry, our main hangout on reddit.
You could also try searching on 'decline of fraternalism'.