It's hard to say how many are registered, as it counts all the people who were on YahooGroups all the way back to 2004, most of whom are probably not here anymore. Certainly the number of people participating is fairly small, at least compared to the busier Facebook group.
Marc, a strange thing is happening to my posts to the safety directory thread. They are emailed to me (I think because I'm subscribed to the thread) but they aren't actually appearing within the thread:
Hmm, yes I am kind of worried about this forum, as I'm not sure there is adequate support, nor are there any plans for improvement. So the whole thing seems to be on life support (hence the opening of an alternate venue on groups.io that is being actively supported and developed).