Polygraph usage

[Lethe]

I am curious if anyone can shed light on polygraph usage in the United States.  That is, how many polygraph exams are conducted in the average year?  And, more importantly, what percentage of them are done for each of the following purposes:

• Federal agency use for pre-employment
  
• Federal agency use on current employees
  
• Non-federal law enforcement for pre-employment
  
• Non-federal law enforcement on current employees
  
• Law enforcement (all levels) on suspected criminals
  
• Law enforcement (all levels) on people on probation, including sex offenders
  
• Use by private citizens or companies, including TV shows

I think even a rough breakdown would help shed significant light on the polygraph debate.  Does anyone have any data on this matter?