(Reuters) - Facebook's initial public offering is likely to set a new standard for how low investment banks are willing to go on advisory fees to win big business. The world's largest online social network is expected to tap public markets for $10 billion in the coming months in an offering that will value the company at up to $100 billion, according to sources familiar with the planned IPO. It will be one of the biggest U.S. market debuts ever, and a prized trophy for the investment bankers seeking to win lead advisory roles. ...