A very relevant history of social bookmarking tools in D-Lib by Tony Hammond, Timo Hannay, Ben Lund, and Joanna Scott.

We are here going to remind you of hyperlinks in all their glory, sell you on the idea of bookmarking hyperlinks, point you at other folks who are doing the same, and tell you why this is a good thing. Just as long as those hyperlinks (or let's call them plain old links) are managed, tagged, commented upon, and published onto the Web, they represent a user's own personal library placed on public record, which – when aggregated with other personal libraries – allows for rich, social networking opportunities. Why spill any ink (digital or not) in rewriting what someone else has already written about instead of just pointing at the original story and adding the merest of titles, descriptions and tags for future reference? More importantly, why not make these personal 'link playlists' available to oneself and to others from whatever browser or computer one happens to be using at the time?

There is also an interesting second part which is a case study of Connotea (reference management service for scientists). Thoses papers have been written by the Nature Publishing Group, the guys behind Connotea.