For what it may be worth, a few thoughts tapping into my day job (web developer) experience...
Facebook is likely to put a lot of people off; as others have already stated, it requires an account which raise issues around aversion to social media and privacy. Similarly Google+ offers "groups" but essentially suffers from the same kind of problems, with arguably an even more invasive use of your online identity. Although its perhaps a little easier to maintain an "alternate" account separate from your real personal one (if you already use google's services that is).Prophpbb
both offer free forum hosting employing the phpbb
forum software. As far as I have heard they're both okay. There is no cost or bandwidth limitations etc, the initial installation and setup work is largely done for you and if you wish to follow the existing Frother's model of "minimal" moderation then ought not to impose on anyone's time too much. I guess it would be the case of somebody/bodies just putting their head(s) above the parapet to start the ball rolling. Links are here:https://www.prophpbb.com/https://www.forumspower.com/
Or... you can opt for a fee-paying hosting option whihc depending upon site traffic could cost as little as £2.75 per month... eg: Siteground (or any other hosting company - I have no vested interests here - but avoid the really
cheap ones like 1&1 etc as they are really crap) which one or two of my "charity/VO" clients use as its perceived to offer a slightly more serious/'professional' impression without breaking the bank... they also offer a wider range of forum software/apps to choose from. See here:https://www.siteground.com/
All the above ideas are offered purely on a "worst case/what if it happens" basis of course.
Jolly good show...