So I did a quick straw poll of my users and so far it’s looking like:
- 18% refuse to touch Facebook for the purpose
- 74% prefer to use my Discourse instance over an equivalent FB Group
- 4% happy to use either
- 4% prefer Facebook
So the message from my main Discourse forum is the vast majority prefer Discourse over FB 
The challenge for me is turning that into an instance that at least covers its costs … 