SAN FRANCISCO (Reuters) - Facebook Inc plans to increase its $2.5 billion credit line to help cover a major tax hit when employee stock awards vest shortly after it goes public, according to two sources familiar with the company's plans. The world's largest social media network, which boosted its borrowing capacity by two-thirds just six months ago, is taking advantage of its strong position to get more financing for its phenomenal growth, the sources said. The sources spoke on condition of anonymity because they are not authorized to speak publicly about such plans. ...