(Reuters) - Just about everyone in Silicon Valley has dreamed of striking it rich with a well-timed investment. Here are the people and institutions that will benefit the most from Facebook's initial public offering, assuming a $100 billion valuation for the world's largest online social network. Some of the percentages below may change due to factors such as what options are exercised and when. MARK ZUCKERBERG Facebook's biggest shareholder and chief executive owns 533.8 million shares, or a roughly 28 percent stake, worth around $28 billion. ...