"Fake news" are widely acknowledged as an important challenge for Western democracies. Yet, surprisingly little effort has been devoted to measuring the effects of various counter-strategies. We address this void by running a pre-registered field experiment analyzing the causal effects of popular fact-checking videos on both believing and sharing fake news among Twitter users (N = 1,600). We find that the videos improve truth discernment ability as measured by performance in a fake news quiz immediately after exposure. However, the videos have not reduced sharing links from verified "fake news" websites on Twitter in the weeks following the exposure. Indeed, we find no relationship between truth discernment ability and fake news sharing. These results imply that the development of effective interventions should be based on a nuanced view of the distinct psychological motivations of sharing and believing "fake news".