Ran across the term stigmergy for the first time (sue me, I've been busy), in a John Robb post at Global Guerillas. Can't say as I'm really into The Global War on Terrorism, but the idea struck a chord in how to think about "communication" with social bookmark systems.
The term apparently comes from biology and the study of complexity generated by lots of simple creatures, ants and termites, using simple signals. Maybe this explains a certain fascination with del.icio.us as opposed to Spurl and Furl. The others are about information management while sharing and signalling seem to be secondary concerns. del.icio.us is all about those little signals and nothing else.
As a smaller world within the Web, social bookmarking strikes me as a sufficiently large, but not unwieldy, space in which to investigate some of these issues. Also, the arena is highly dynamic and a bit more amenable to automation then average Web crawling.
As to blog indices (blgodex, Technorati, Popdex) maybe their popularity (fading I might add) is a yearning for some kind of stigmergic totems.
Update: I don't claim any particular brilliance in attempting to make these connections. I'm sure a few have trod this path before me.