I dimly recall having read a great deal about "online communities" (maybe I'm thinking specifically of something I read in Sherry Turkle's so-so book Life on the Screen ) where it was assumed that what you needed that made you a community was that people can't totally come and go at the drop of a hat. I'm starting to think that while this may be a common (or emergent) attribute of a community, it's "putting the cart before the horse" to consider it necessary, much less something that should be on a checklist of things you put together in order to make a community spring out of nowhere.
Instead, maybe it's better to think of "social capital" as more important -- i.e., that people form a community as a way to share useful information, or other tools. If you look at it that way, CPAN and various Perl forums (like this) are communities where people benefit from social capital in an informational form -- where module code, or whatever, is the information economy equivalent of the farm tools in "Social capital is productive, since two farmers exchanging tools can get more work done with less physical capital." (And programmer time/effort is the equivalent of physical capital there.)