Re: What is the biggest forum on bbPress?
There’s an excellent Private Forum plugin:
And the ability to create and define custom user groups to access a particular forum:
You mentioned a few social networking type plugins… have you checked out buddypress, the social network extension to WordPressMU?
It might be overkill for what you’re looking for, but at least some of the feature/plugins do exactly the sorta things you mention (galleries, pm, etc.).