Personally, I interpreted the word "challenge" as more of a literary tool than antagonistic comment toward the moderators. Words can have different meanings based on context.
I believe the information you seek would be interesting, but would also be difficult to truly learn. There are too many confounding variables.
For example, I never put personal information on the web, including gender, age, "race," sexuality, religiosity, etc. Many people make a similar, conscious, decision to remain as anonymous as possible online. Personally, I enjoy observing what "cues" people use in attempts to ascertain the previously mentioned demographic data. (Sidebar: It is to their credit that the board administrators do not require that sort of information.)
At the top of the page is the number of total members. I know some BBS software will show number of users active for the past x number of days, but I don't think those automatically break it down for a yearly totals.
However, if we had the number of active users per year, it may reflect the popularity of a BBS, how well known it is, and how involved those users are more than reveal general user trends for a particular product. I'd briefly been on one automotive BBS that had a mass exodus to a new board that was operated by, shall we say, more level-headed people than the former. So the numbers would have been quite confusing, and unrepresentative, in a case like that.
Numbers could also be inflated because of a thread where people may feel compelled to join a board just to comment, like the Herculiner thread (may not be suitable for young children, though not obscene) of yet another automotive board.