The tendency of netizens to form self-contained groups is strengthened by what Eli Pariser, an internet activist, identified five years ago as the “filter bubble”. Back in 2011 he worried that Google’s search algorithms, which offer users personalised results according to what the system knows of their preferences and surfing behaviour, would keep people from coming across countervailing views. Facebook subsequently became a much better—or worse—example. Although Mark Zuckerberg, the firm’s founder, insists that his social network does not trap its users in their own world, its algorithms are designed to populate their news feeds with content similar to material they previously “liked”. So, for example, during the referendum campaign Leavers mostly saw pro-Brexit items; Remainers were served mainly pro-EU fare.