For better or worst, Fb has become the de facto place for cruising sailors to share information about different regions of the world. Tips, alerts, advice, questions, etc. I sail the world and there is no other place for groups quite as good for finding the information we need. There’s a niche group for every area around the world full of people sharing advice and answering questions. The good groups have great moderation and quality content.
> Discord, Whatsapp (huh!), Signal
where nothing is searchable, linkable, or otherwise legible. That's a tradeoff. It has some upsides, but downsides as well.
(granted search has been struggling to not suck in general lately, facebook among others has joined the campaign against legible links, so losses are taking place in web environments anyway)
Moving a community is much much harder than you're making it out to be. Especially when there's a long history of content and - most importantly - trust between members.
Asking people to re-learn the new modalities and UIs and where everything is etc.. particularly for a less technical crowd.
They can move to Discord, Whatsapp (huh!), Signal, Reddit or some sailor forum.
If FB disappeared they could reroute.
Bigger things are disappearing and we are going to be fine*. E.g. much world trade with the US.