Elon Musk's net worth fell along with Tesla stock on Friday.AP Photo/Alex BrandonTesla debuted its prototype Cybercab at Thursday's "We, Robot" event in Los Angeles.Tesla stock fell more than 9% on Friday, reducing Elon Musk's net worth.Though his net worth dropped by about $15 billion, Musk remains the world's richest man.Tesla's robotaxi event, at which the company debuted the long-anticipated prototypes for its Cybercab and Robovan, failed to impress many investors and Wall Street analysts.