title_body: What are some real world data on the numbers of speakers of constructed languages? What are some real world data (e.g., from census data, membership data, visitors of conlang related events, other estimates) on active speakers of constructed languages?
upvoted_answer: There is some basic data at Ethnologue for Esperanto. 
  The Font of All Knowledge lists some statistics for other invented languages (Hungarian &amp; Russian census data).
  
  
  Not much, but a (small) start.