In reply to Harold Jarche » A few good books.

Both/AndGood points all around. I guess what I'm trying to say is that what we view as virtual communities are sometimes limited by our technology. The human side is pretty clear and most of us would agree that a "community is the people, not the tools". That's why I'm interested in the effects of blogging versus CoP's (as limited by web tools that provide a shared space to which a member has to go). Blogging, while not for everyone, is more persistent and I think that communities of bloggers are dynamic and therefore more enduring. The barriers to entry and exit are very low.
What Elgg Learning Landscape offers is a new metaphor/model and Elgg has opened my eyes to a different way of supporting online communities. By removing the walls around the community you make it easier to come and go. A "community" as defined in Elgg is the same as a person. You don't join a community, you connect with a community, as you would with a friend (à la FOAF). This seemingly minor change means that you no longer have to go to the community. The community comes to you.
In our Elgg implementation we also added an automatic wiki each time a community is created. Because wikis are multi-authored, an individual doesn't really need one (each person has a blog and file repository), but every community gets a place for its members to co-author web pages. Elgg also lets you form groups, which are separate from communities, so that you can annotate any document or blog post as public, visible to only you, accessible only to members of a certain group or only members of a certain community. Had I not started with Drupal (with almost 700 posts) I would happily switch my blogging and collaboration to Elgg. I have not found anything else this easy and flexible. Really, the magic is in the underlying model.