Why Facebook Showed You That Ad for the Candidate You Hate

You may be a reliable Democratic voter in a solid-blue city. Maybe you have a graduate degree; maybe you’re a member of an ethnic or religious minority; maybe you are a woman. Any of these would make you a likely Hillary Clinton supporter. So why did you just see an ad for Donald J. Trump’s presidential campaign on Facebook? The New York Times has collected an extensive database of political Facebook ads, and data about how they are targeted, from our readers as part of our Political Ad Tracker project. Microtargeted online advertisements can be a powerful tool for political campaigns to tailor a message to specific people they are trying to reach. But the reason you saw a particular campaign ad on Facebook may have nothing to do with your political views, or even your demographic profile. It could depend on whether you live in a swing state and on your internet activity — or be practically random.