Facebook’s essential threat to democracy isn’t that Mr. Zuckerberg will intervene on behalf of his preferred candidate — it’s more fundamental than that. Mark Zuckerberg need not intervene, because Facebook, the platform, will do so instinctively. With its algorithmic mandate of engagement over all else, Facebook has redefined what it means to be a good candidate — and provided a distinct natural advantage to those who distort the truth and seek to divide.