What constitutes a 'meaningful' number is subject to difference of opinion. BB cited several different sources for his estimate. However I strongly suspect that BB made absolutely no allowance for overlapping memberships in his estimate.
My own 2 forums number 1000. There are at least 3 other major FZ forums.
There is much overlap.
And note that Marty's people are mostly not included.
Then there is 2-3000 Russian FZers.
Its thousands not hundreds.