Actually, since you mention it, this is pretty consistent with how things have been here for always.
People’s lives change, their interests change with them. What was appropriate or timely or interesting or meaningful to them today is not so tomorrow and they’re on to something else entirely next week. That’s the nature of life.
So it is with the SDMB. Some find us and stay forever. Most come for a while, read, participate and move on to other things or get involved in different activities. They have a new love interest or they just bought a house or there’s a new baby or they’re on a different schedule. Sometimes people read only at work and then one day the Straight Dope is blocked on the site and they just never get around to doing this at home. People graduate from school and find less free time afterwards, or they take on new projects that give them less ability to sit and read.
The reasons are varied, but people move on.
In rough terms (not going to quote numbers at you here), the number of new people to the Dope equals or surpasses the number of people who fall away. Page views have risen and continue to rise. This is a pattern that goes back even to our AOL days and would seem to indicate that there is a small dedicated core of more or less permanent members and a much larger and more fluctuant population of sometimers who come in and stay for a while and drift away.
We used to get little surges of participation when we were listed on the AOL Welcome screen, to give just one example; people found out about us and ran to check us out. Same with Straight Dope on A&E; we’d always get little bumps on Sunday nights. It’s still the same today, as any mention elsewhere in cyberspace or anywhere else drives people to come see what this is about.
I have seen nothing on the board over this time to indicate any difference in this pattern and that includes subscriptions; in fact, the retention rate on subscriptions seems to bear that out, it’s more of the same.
Of course we hope that everyone likes what they see, becomes a subscribing member, and participates fully in the entire Straight Dope experience. That’s not always what happens, but that’s what we’d like, anyway.
So the kind of pattern you see with guests replacing members, it’s the normal course of events here and always has been.
Not a statistician.