Esperanto speakers
I did a little research--what the heck, my browser was already open--Esperanto is spoken fluently by about two million people (other than casual users who know a few phrases or words):
How many people speak Esperanto?
Professor Sidney S. Culbert of the University of Washington, Seattle, USA, has done the most comprehensive survey on language use ever attempted. He has conducted interviews in dozens of countries around the world and tested for "professional proficiency", i.e. much more than just "hello, please, goodbye".
Based on this survey, Prof. Culbert concluded that Esperanto has about two million speakers worldwide. This puts it on a par with "minority" languages such as Lithuanian or Hebrew.
Quoted from: "Esperanto: The International Auxiliary Langauge"
URL:
http://www.geocities.com/quetl/esperanto.htm
I am afraid Klingon is far behind:
There are Klingon language conventions held annually and Okrand estimates the number of those who can fluently speak Klingon is at least 100, and growing.
Source: "Klingon language gaining acceptance on Earth"
URL:
http://www.exn.ca/Stories/1996/11/12/01.asp
(Mark Okrand invented Klingon grammar, so he should know who is fluent or not.)
Of course, you could quibble forever over who is or is not fluent enough to qualify as a fluent speaker.
A lot more people have heard of Klingon than esperanto, perhaps.