Mass Political Information on Social Media: Facebook Ads, Electorate Saturation, and Electoral Accountability in Mexico

Social media’s capacity to quickly and inexpensively reach large audiences almost simultaneously has the potential to promote electoral accountability. Beyond increasing direct exposure to information, high saturation campaigns—which target substantial fractions of an electorate—may induce or amplify information diffusion, persuasion, or coordination between voters. Randomizing saturation across municipalities, we evaluate the electoral impact of non-partisan Facebook ads informing millions of Mexican citizens of municipal expenditure irregularities in 2018. The vote shares of incumbent parties that engaged in zero/negligible irregularities increased by 6–7 percentage points in directly-targeted electoral precincts. This direct effect, but also the indirect effect in untargeted precincts within treated municipalities, were significantly greater where ads targeted 80%—rather than 20%—of the municipal electorate. The amplifying effects of high saturation campaigns are driven by citizens within more socially-connected municipalities, rather than responses by politicians or media outlets. These findings demonstrate how mass media can ignite social interactions to promote political accountability.