You're less likely to check facts in a social media crowd than when alone.

Jun et al. (open source) make a stab at characterizing the societal problem of "fake" news:

Significance

The dissemination of unverified content (e.g., “fake” news) is a societal problem with influence that can acquire tremendous reach when propagated through social networks. This article examines how evaluating information in a social context affects fact-checking behavior. Across eight experiments, people fact-checked less often when they evaluated claims in a collective (e.g., group or social media) compared with an individual setting. Inducing momentary vigilance increased the rate of fact-checking. These findings advance our understanding of whether and when people scrutinize information in social environments. In an era of rapid information diffusion, identifying the conditions under which people are less likely to verify the content that they consume is both conceptually important and practically relevant.

Abstract

Today’s media landscape affords people access to richer information than ever before, with many individuals opting to consume content through social channels rather than traditional news sources. Although people frequent social platforms for a variety of reasons, we understand little about the consequences of encountering new information in these contexts, particularly with respect to how content is scrutinized. This research tests how perceiving the presence of others (as on social media platforms) affects the way that individuals evaluate information—in particular, the extent to which they verify ambiguous claims. Eight experiments using incentivized real effort tasks found that people are less likely to fact-check statements when they feel that they are evaluating them in the presence of others compared with when they are evaluating them alone. Inducing vigilance immediately before evaluation increased fact-checking under social settings.