Yes I have. It works for tech communities. It works for topic discussions. It works for many things. What it would completely fail for is a worldwide group of people who have a lot of personal connections between them, organise and announce semi-public events (100s a year), have local groups (1000s), and share a lot of information within one of those relationships. Also ~100% of them are already on FB and successfully do all of the above.

So again, no. Monopoly is bad, Facebook is bad/good depending on use case, sure. But this is not possible to migrate to a forum. Very simplified situation exists for developers - with the personal part reduced and with very limited local groups you have this information spread over twitter / local boards / meetup.com / etc. anyway. If I wanted to check events in some area, it's actually not trivial. It would be also a complete pain to centralise. You know what I do for the other use case? Post to facebook, share it in a group that's approximately in the same area in the world, and I get all information I need in a few minutes / hours: links to specific groups, specific events, and if needed I've got FB messenger contact to people who posted it.