A link between populism and social media is often suspected. This paper spells out a set of possible mechanisms underpinning this link: that social media changes the communication structure of the public sphere, making it harder for citizens to obtain evidence that refutes populist assumptions. By developing a model of the public sphere, four core functions of the public sphere are identified: exposing citizens to diverse information, promoting equality of deliberative opportunity, creating deliberative transparency, and producing common knowledge. A wellworking (...) public sphere allows citizens to learn that there are genuine disagreements among citizens that are held in good faith. Social media makes it harder to gain this insight, opening the door for populist ideology. (shrink)