I've been knocking about social media since the '80s and you always get a proportion of lurkers, who read but rarely write. I suspect that with this sort of forum you will also get people who join just to see what it is about and don't come back because it wasn't what they expected or they satisfied their curiosity. I'm more used to email lists, having run a few, where people actively quit to save their mailboxes from being filled if they aren't interested.
Does it matter? I don't think so. I doubt if it takes more than a few dozen bytes to store a user registration. If security is a worry you could disable passwords after a while, which would let people rejoin if they wanted to.
For those who stay this is certainly one of the best online groups I have encountered and the only web based forum I have stuck with. I'm very happy to be here.
