I tend to agree that as a discussion format, Facebook doesn't work as well for me as a discussion forum.
I have seen two instances where Facebook proved helpful.
The first was a closed group "Bike Touring Americas: Alaska to Patagonia". I believe this initially was a discussion list set up by people interested in the topic. However, as the principals completed their rides, eventually this became stale and was replaced by a Facebook Group. Where I've found it useful is someone posting a quick informational question e.g. "any experiences crossing the Peru/Ecuador border recently". All this took a hit with Covid but still helpful as a way of getting somewhat recent information. iOverlander is a different format to get sometimes similar information.
The other is a smaller closed group, "Riding Every Day in November"; that started as an informal challenge by a friend of mine to his cycling friends to challenge individuals to get out and ride - particularly in Pacific Northwest of US where many were based. This is relatively small and supportive group - where culture and perhaps moderators keeps it positive and supportive.