Can't we leverage this lesson then? If smaller groups can be integrated into the larger one with enough time, then the answer is straightforward. You can have very large internet forums, but new users need to be slowly integrated into the 'culture' piecemeal.

Create a good culture, then break it out and parallelize it. New users get slotted into larger groups to learn the culture, then they can be sent on their way into the larger culture.

Twitter Training Wheels?