social bookmarking portal lets users share expertise on some subject; room for author's comments and explanations; more discursive than delicious; less writing than a blog; see "Visual Thinking," also "Ephemeral Films"
Firefox extension offers an addictive way to search via categories or keywords or happenstance; you train it, with thumbs up or down on sites, so it learns your tastes; I prefer searches on keywords so far, and have had little desire to "network"
You sign up, enter the name of a book, then find out what other folks who liked that book also liked; a tag cloud for the book leads to other books on this topic. Quite addictive, and I just used it to look at the info. around one book.