There were 1000s of members which were clear spam accounts. After deleting most if not all of the spam, I went and deleted everyone who had 0 posts.
Some legitimate users may have gotten swept up in that. If so, my apologies. That's what you get for exclusively lurking.
I might get rid of all these categories and put them all in one forum. There were calls to do that long ago but now it seems like it might be time. But first I need to upgrade the forum from 3.0.1 to 3.0.5. So look for these things to happen soon. So if the forum is down for awhile, you know why.