I left a comment over on Andre's great post on social media training wheels. Rereading the comment, I wanted to post it here as a fully-fledged blog entry.
We're at a point where the exact tool selected really doesn't matter very much anymore. Any of these communities can be built in pretty much any open source web platform. The key is that it's open source, so it's easily modifiable (or at least modification is _possible_), and the ownership of the software and community is located within the institution, rather than at a corporate headquarters.
That said, most of the most active communities on UCalgaryBlogs.ca are closed to outsiders – walled gardens for use by the class. And I'm fine with that. The goal isn't to publish content to the open internet. The goal is to engage students, in creation, discussion, and reflection. If they need a walled garden to do that effectively (and there are several excellent reasons for needing privacy for a community) then so be it. If they'd like to do it in the open, that's just a checkbox on a settings page.
That option isn't available for users of The Big Commercial LMS Platform. If it's in an LMS, it's closed. End of discussion. And people only gain experience in using the LMS, in farming for Maggie.