The following commentary comes from an independent investor or market observer as part of TheStreet's guest contributor program, which is separate from the company's news coverage.By Richard Levick and Kathleen Wailes NEW YORK ( TheStreet) - Activist investor Daniel S. Loeb just claimed another corporate scalp. Tuesday's announcement that Yahoo Chairman Roy J. Bostock and the company's three longest-serving directors will not seek reelection represents Loeb's second major victory in as many months. In January, Yahoo CEO and co-founder Jerry Yang stepped down amid rampant speculation that Loeb was plotting a proxy fight aimed at forcing a management shakeup. Yang's ouster and the subsequent board overhaul were no small feats, even at an underperforming company such as Yahoo. Notch another win for the activists. More are sure to follow.