My mental model of that question is: how would anyone know if an ontology was used by something? One cannot have a search facet in any engine that I'm aware of to search by namespace qualified nouns, and markup is only as good as the application which is able to understand it
They are _used_ very extensively, but not in web pages. Many industry domains rely heavily on linked data and ontologies.
It's true that they are less commonly embedded as semantic data in web pages. There's a real bootstrapping problem there: no reason to embed data if no tools will read it, no reason to build tools if there's no data to read.
