This thesis addresses two paradoxes: (1) why empirical studies find that fake news represent only a small share of the information consulted and shared on social media despite the absence of editorial control or journalistic norms, and (2) how political polarization has intensified even though users do not appear especially receptive to fake news. To investigate these issues, two complementary studies were carried out on Twitter and Facebook, combining quantitative analyses of digital traces with online observation and interviews. This mixed-methods design avoids reducing users to single reactions to identified fake items and instead examines the variety of practices across different interactional situations, online and offline, while recording socio-demographic traits. The first study mapped users who shared at least one item labeled fake by fact-checkers in the French Twittersphere. The second used a corpus of items flagged by Facebook users to study reactions to statements whose epistemic status is uncertain. Three main findings emerge. First, sharing fake news is concentrated among a limited group of users who are not less educated or cognitively disadvantaged but are more politicized and critical of institutions; owing to their high activity and prolific sharing, they can help set the agenda for their political camp. Second, exposed users can deploy varying forms of critical distance depending on their social position and the interactional norms of the situations they inhabit: either discursive caution (prudence énonciative) or interventions ('points d'arrêt') that express disagreement or corrections. Third, these forms of critical distance seldom yield genuine deliberative debates or agonistic pluralism; rather, they often produce dialogues of the deaf among a small, particularly active minority.