Something interesting is happening among shareholders of the energy giant ExxonMobil, which may usher in a new era in activist investing. A new activist hedge fund, Engine No. 1, is pushing widespread reform at Exxon through its “Reenergize Exxon” campaign. What makes this campaign extraordinary is Engine No. 1’s tiny size — it holds only a $40 million stake in the multibillion dollar company. Yet there are strong signals that Engine No. 1 is in position to reorganize and reform Exxon from the very top of the company.