I like the idea behind the semantic web and if everyone had the bandwidth to commit resources to it, it would definitely add additional abilities to the web. But, in my experience, the time spent on adding RDF support wasn't worth the adoption or limited gains in site usage.

What are its usage metrics in your site(s)? Does it appear that there is significant growth in its adoption?

Facebook's has offered RDF for a while. I've not heard of its use outside of academic circles.