People are downvoting this, maybe because of the fact-of-the-matter'ness, but I think this is probably true, coming from someone who despises Facebook. Especially for certain age-groups, Facebook is one of the few social networks you can assume people have logins on, and (maybe especially) niche communities need to lessen friction to get people involved.
I don't know, I've never tried to create a public forum for this kind of thing, but I'd bet you'd get more activity out of facebook than some bespoke web forum or IRC/Matrix/what have you.