Greylock Partners, a top-tier Silicon Valley venture firm, said on Tuesday it has hired a new investing partner in Josh McFarland, who will be leaving his executive post at Twitter Inc. McFarland, who has been vice president of product at Twitter, is just the latest executive to leave the struggling social media company this year. Four senior executives left San Francisco-based Twitter in January, and another two executives departed in May amid the company's tumultuous efforts to find new products and features to grow its user numbers.