In October, Twitter co-founder Evan Williams handed Costolo -- a onetime improv performer -- the top spot, a tacit acknowledgment that finding a way to make Twitter profitable has become the top priority. The microblogging service has grown to 160 million registered users -- 65% of the Fortune 100 had active accounts by this summer -- though monetizing that base has been tough. This year the company hired Adam Bain from News Corp. and Ali Rowghani from Pixar to divine new revenue streams. Perhaps the company's new ad platform, Promoted Trends, will help.
NEXT: 25. Ratan Tata