A love the term usable exhaust which I ran across at Hypergene MediaBlog. The phrase captures the essence of the more academic, social navigation, in an accessible fashion with a dash of potential activity thrown in.
The subsequent analysis is a bit lacking though.
First off, Chris Willis and/or Shayne Bowman correctly point out that tagging isn't magic pixie dust, but as implemented tagging is extremely important as usable exhaust. In both del.icio.us and Flickr each tag emits an RSS feed, which makes it easy to monitor a topic's exhaust. Even better, in both systems you can watch a given person's tag, so you can actually focus on people (also groups in Flickr) that you care about. Plus, you can discover new tags through the feeds as well. The del.icio.us popular page is probably a bad example as it often includes stale links and for many del.icio.us users the popular URLs aren't all that useful.
Also, the veneration of APIs is undeserved. In both systems, the number of applications that use the API, introduce users to each other, and make it easy to find new, interesting content is vanishingly small. Of those I bet they are used by a relatively small minority of the community. Heck, about the only thing the del.icio.us API is really good for is backing up your bookmarks, since you can only see your own and can't get any global information.
I could be convinced that the Butt-Brush factor is a real issue, but I'd counter with the amazing number of ways social connections can be made in Flickr if you choose to be public: groups, group photo pools, comments in groups, comments on photos, favorites, interestingness, contacts. Also, on every photo the tags, sets, and pools it participates in are visible and navigable, so the more exhaust you generate the more you grease the skids for other people to get sucked into Flickr. Finally, there's a white hot core of community on Flickr that probably generates the most traffic through socializing and providing the best content. Luckily Joe Just Get Photos Online can ease in with little hassle, but he makes up the weak periphery.
The authors are right that RSS is easily overlooked. They did it themselves. Again, RSS isn't just an add-on, it's an essential mechanism by which people can monitor the activities of others. It's how people see your exhaust!
Other than that, great post!