I guess another issue is the low barrier for entry for an FB Group.
One of my instances is a residents community … there’s a parallel FB group and in one amusing turn of events, when a huge bit of bad vibe and argument was sparked on the initial FB group, someone started a rival FB group to accommodate all but the persona-non-grata. So there are now two FB groups covering the same scope … Of course with Discourse you have more active Moderation, which surely would have controlled this, hopefully, but I digress.
The point is you can set up an FB group very quickly & cheaply, whereas with Discourse you know the options - the solution is by far and away better, but it takes more work and/or money. A frustrating advantage they have in that respect.