It's called Reddit.
Bear with me here, I realize this is probably an unpopular opinion.
On Reddit, all content is open -- I don't think I've ever seen a 'closed' sub-reddit available only to members. I'm not sure it's even possible.
Yet, in spite of all communities being effectively open, most sub-reddits have a strong sense of community, and are very active. Personally, I really enjoy proggit and r/math for this reason -- active discussion, community moderation, and a strong sense of community.
The next time you hear the phrase "It doesn't scale" in reference to something, ask yourself why it needs to. In some cases it might not.
Centralised approaches (e.g. websites on the open internet) obviously need to scale. Facebook is a public website. And it's on the open internet, for anyone with an account to see and ready to be hacked by those with skill who do not need accounts.
I always come back to this: How many people do you really need to be reading your profile? Friends, coworkers, family, ... potentially anyone, anywhere in the world with a computer and an internet connection? It just does not make sense.
As for shutting it down: The code is open source, so while it'd be hard to unite around a common replacement, if they tried to shut it down it would not be the end.
I think part of what has happened to these dwindling forums is that they are functionally out-dated, and a bit too insular. -It takes too much time for new people to get up to speed on most boards. (dont forget Hubski, btw. :))