Just an observation, but here are some very random statistics.
I used Wayback to check “active members” on the SDMB main page. I don’t think that the “active members” feature of vBulletin has always been enabled, so the first number I can find is from…
5/23/2011, 11,167 active members
9/1/2013, 6289
9/28/2015, 5410
2/20/2017, 4489
6/4/2017, 4063