I don't know, I find that in smaller web communities I tend to participate more, because I know my voice is more likely to be heard. And if everyone knows everyone than people would be more easily remembered for making good comments. That would give them more of an incentive to be helpful and constructive. So the knowledge loss might not be as great as it would seem.

And communities could still be visible to everyone, even if only members of that community can participate. Then if there's a really good comment in one community, someone from another could just copy and paste it over.