The term echo chamber refers to the way that the algorithms used in social media ( a form of recommender systems) may mean that users only see posts that amplify their existing beliefs giving the false impression that their views, even if extreme, are widely accepted, and increasing societal division.
Used in Chap. 19: page 315; Chap. 20: pages 335, 345; Chap. 24: page 402
Also known as filter bubbles