CommonCraft Video has the best training materials I've seen on social collaboration tools. See for instance Doug Cornelius' KM Space posts on Wikis and also on RSS feeds, "RSS in Plain English."
Their next video educates about the power of social bookmarking:
Last month in Pivoting In Delicious I dug down a little bit more into the social collaborative aspect of the tagging than Lee Lefever does here. In particular, I explained how you can examine others' Delicious tags for a particular URL, and, how useful that social context can be.