Third Point Management, Daniel Loeb’s hedge fund, has disclosed that it has acquired a 5.15% stake in Yahoo and hinted at a proxy challenge if changes were not made on the Internet company’s board.  The move increases pressure on Yahoo's board to make serious changes after firing CEO Carol Bartz. With the stake, Third Point becomes the third-largest outside investor in Yahoo, after Capital Research Global Investors and Capital World Investors. In a letter to Yahoo’s board, Loeb wrote that “it is time that certain members of this board were held accountable for its past failures and their individual roles.”  The letter calls for the resignations of Yahoo’s chairman, Roy Bostock and three directors, Arthur Kern, Vyomesh Joshi and Susan James. He added that while he was pleased with Bartz' firing, “we fail to understand why this decision was so long in coming given her abysmal performance over the last two and a half years.” Mr. Loeb says that Third Point is “eager” to present candidates for the board and it is prepared to propose its own rival slate at the company’s annual meeting next year, “should it become necessary.” While Third Point has not been as vocal as an activist investor of late as it has in years past, it does have experience in mounting proxy challenges. In 2006, the hedge fund won two seats on the board of coal producer Massey Energy.  [DealBook, 9/8/11]