Robert Kaye - High order bits and Ontologies:

"Then later in the afternoon, Clay Shirky talked about the difference between ontologies and folksonomies in his "Ontology is Overrated: Links, Tags, and Post-hoc Metadata". With his usual flair Clay delivered a great overview of classic ontologies and all the issues that limit their usefulness on the Internet. [...]

Clay went on to outline the conditions under which classical ontologies can thrive:

* Domain: small corpus, formal categories, stable entities, restricted entities, clear edges * Participants: Coordinated users, expert users, expert catalogers, authoritative sources

In a nutshell, ontologies work best in small and controlled environments where experts are using the system. Unfortunately, the Internet is the the exact opposite of all of these. And thus, argues Clay, ontologies are not suited for the Internet. Fortunately, the Internet has brought us a solution to all these problems in the form of Folksonomies."