From Arika Okrent’s In the Land of Invented Languages: of the 500 invented languages that Okrent profiles, the two most commonly spoken today are Esperanto and Klingon.
This is probably one of those things that everyone [in nerd land] already knew but me, but still. Wow, Trekkies. I was always a Star Wars girl, but color me impressed. (Also, this makes me wonder how the number of Klingon speakers compares to the number of people who would declare Jedi as their religion.)