I'm sure the author is right to some extent about the rise of Twitter and Facebook being responsible for the decline in social bookmarking, but I think it has a lot to do with the fact that good curation is almost always better than a system that is controlled by upvoters, linkers or tweeters.
"Just watch, Web 3.0 will be something called Good Editing." Drew Curtis of Fark http://www.ideagrove.com/blog/2006/07/media-orchard-intervie...