Personally, I don't trust the member figures shown on most message boards.
They list the numbers of people subscribed which includes:
1.) People that signed up and left without removing their id's(if the Software even allows that).
2.) People that signed up but never posted.
3.) People that forgot their login/password.
4.) People that are subscribed but are browsing the board without bothering to log in.
5.) etc.
Plus, after every major software/GUI change you will see a drop in participation/postings.
Which might pick up or not, IME, it tends to drop.
As an average less than 10% of the subscribers are actual participating posters(if you are lucky).
One Board I know was down because the admin happened to be stationed in Iraq during the crash for a few weeks. It nearly killed the board.
Same can happen when a bunch of regular posters leave due to "political/admin" issues.
Seen quiet a few boards to go from 100+ messages a day to less than 10 a month, even though they record 500+ members.