Joshua Schachter is the inventor of del.icio.us, the most well-known social bookmarking site. Techology Review has his story here. I like this explanation of "folksonomy":

What del.icio.us's users were creating--without necessarily knowing they were doing so--was what technology blogger Thomas Vander Wal has dubbed a "folksonomy," a flexible system of organization that emerges organically from the choices users make. We're all familiar with the alternative, the kind of rule-bound, top-down classification scheme that Internet theorist Clay Shirky calls "ontological" in nature. The Dewey decimal system is an example: every object is assigned its place in a hierarchical system of organization, and every object is defined as, ultimately, one thing: a book goes in one place in the library and nowhere else. In a folksonomy, by contrast, definitions are fuzzier. With del.icio.us, the same Web page has many different tags, which often aren't even related to one another, and no explicit rules are being followed. Web pages are therefore listed not in one place but in many places, and sometimes pages aren't quite where you might expect them to be. So folksonomies are messier than "ontologies" are.

What del.icio.us has shown, though, is that folksonomies' imperfections are outweighed by their benefits. In the first place, folksonomies are dynamic rather than static. A Web folksonomy thus allows us to reclassify content according to our changing interests. An academic paper that's interesting today might be equally interesting a decade from now--but why it's interesting, why people care about it, might be very different. A traditional categorization system has a hard time dealing with this: once the essence of an object is defined, it's supposed to be defined for good. In a folksonomy, the reclassification happens almost automatically--as people start tagging the paper with new, more relevant tags, for example. Web folksonomies are also better at capturing the multiple meanings and uses that a given site has, rather than constraining the possible range of meanings.

The ability of tags to fluidly organize data is why I indentify it as a key technology for creating the datasphere.