Ok, fair enough. Thanks for the info!
Yes, the forum is running on phpBB (3.0.x) but we are using prophpbb.com as hoster. They provide a pre-configured, ready to go phpBB hosting where we cannot install any plug-ins ourself (only what they provide globally to al their customers).
We looked at switching to Discourse but at the moment it's too expensive/complicated to host and given the amount of forum users we have left it might not be worth it.
We may reconsider at some point in the future though if Discourse get cheaper to host, easier to convert to and the user base shifts back from Facebook.