About 5 or 6 years ago. Prolog is still used so if you are enthusiastic you might do an open source project in Prolog to draw attention of other people using the language.
You do Clojure, right? I've been working on a Prolog in Clojure, https://github.com/clojure/core.logic. Hoping to get stratified negation and Definite Clause Grammar support soon.
I have always longed to be paid to work in prolog since I was introduced to it in college.