By nearly any measure, Facebook Inc. is an extraordinary success. Its market capitalization exceeds $500 billion. Its user base outnumbers all but one continent. Mark Zuckerberg, its chief executive officer, is one of the world's wealthiest people. No one should begrudge Facebook (or Zuckerberg) this success. Yet as it begins to play a more central role in American media and politics, the public has a right to expect it to accept the responsibilities that come with its growing power and influence.