In reply to Harold Jarche » Born in a storm.

Inside / outsideBruck's 90% strikes me as metaphoric, though it's a metaphor I'd use. Of one list I've belonged to for over a decade, I often say that 90% of its content is worthless but that I can't predict when the 10% I find useful will arrive. So I hear him saying simply "most of the time this stuff doesn't endure."
Like residential communities or like single-location workplaces, "conceptual communities" have their membership change over time. The arrival or departure of one individual doesn't usually make that much difference (though it can), but gradually there's a shift in interests, emphasis, outlooks.
I tend to agree with Visitor that sometimes the boundaries are helpful. Instead of "boundary," consider something like "common interests" or "shared context." I might want to participate in a community of practice related to job aids and performance support, say; very detailed contributions about how to edit graphics (even if used in performance support) might get tangential to the community's primary interest. Sometimes you want to make a sandwich, not learn how a three-star chef sharpens the knife he'd cut it with.
Another case of both/and, to be sure.