That would be an interesting feature, but it's also an incredibly complex one. I doubt the core team will ever work on something like this, however we might see a community extension that implements something like this one day.
It's already very complex to make good moderation tools for moderators, it's even more complicated to make good moderation tools for users to moderate each other in user-managed communities.
I feel like usually a forum is what would replace a Facebook group, so you wouldn't want to completely separate the discussions between sub-groups since the forum is already for a specific topic.
For most simple cases, groups and permissions can be used to create different areas and access levels in Flarum without making things incredibly complex.
I can see some advanced use cases for situations where you might want users to have separate work groups or projects. But if the goal is to make a general social platform there will likely be a lot of additional features required.
