And more recent numbers for science, engineering, and else:
And Jordan Weissmann at the Atlantic also looks at the job market for PhD candidates awarded in the US and then considers how foreign, visa-holding PhD candidates fair in contrast to Americans and Green Card holders. These show that approximately 30 to 40 percent of PhD candidates do not have a job or assignment of any type lined up at time of graduation. (Similar charts one year out would be more informative.)
Jim Austin argues that Mathematics is the best PhD to get while Noah Smith says it is Economics.