Policymakers increasingly ask Internet platforms like Facebook to “take responsibility” for material posted by their users. Mark Zuckerberg and other tech leaders seem willing to do so. That is in part a good development. Platforms are uniquely positioned to reduce harmful content online. But deputizing them to police users’ speech in the modern public square can also have serious unintended consequences. This piece reviews existing laws and current pressures to expand intermediaries’ liability for user-generated content. It discusses three ways that poorly designed laws can do damage—to First Amendment-protected online speech, national security, and the economy.