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.
Defined on page 507
Used on Chap. 19: page 459; Chap. 20: pages 490, 507; Chap. 24: page 585
Also known as filter bubbles