The Human Layer
If the only thing the bookmark-sharing sites did was host a place on the web where people could store, organize, and share their favorites, they'd be providing a truly useful service. But people do more on these sites than stash URLs. They describe the sites they're bookmarking, comment on their content, and tag them with telling search terms. Other users effectively rate those bookmarks by adding them to their own collections or leaving them out. All of which introduces a distinctly human layer to this process.
People have culled the bookmarks collected on these sites from the web. You might say they've been handpicked. Users of social bookmarking services get to take advantage of the expertise and interests their friends and family, and potentially thousands of people they've never met. You could also call these services peer-to-peer search engines, because they allow you to search for subjects by user-generated keywords and find results recommended by the community.
Which is not to say that technology is completely absent from the bookmark-sharing equation. These services often add their two cents with automated tools designed to rank a website based on the number of times users have bookmarked it, for example. Popular bookmarks rise to the top, and unpopular ones drift to the bottom. And a good search engine is essential. Users search the combined favorites of hundreds of people by keywords, user names, and popularity.