Hubzilla and Friendica both have federated forums and GNU-Social has a marginal implementation.

There are a few issues with some networks seeing comments from members of other protocols which their projects don't personally implement. So I can provide a forum on Hubzilla that can be used by Mastodon folks and/or a forum that can be used by Diaspora folks and all is hunky-dory (and Hubzilla can see conversations by everybody involved), but providing a forum that can be used by both of these other projects simultaneously would require each of them to understand each other's protocols.

Otherwise their network won't see comments and likes from the other network.

It's possible to work around this, but it's a huge development effort and it wouldn't be necessary if other projects and protocols were actually open to cross protocol federation; but mostly because I'm working on more important things at the moment.