Despite the growth of social media as a primary channel for news consumption and for other political activities, little empirical work has established a causal link between exposure to political content on social media and public support for salient policies. More importantly, not much research has been conducted on how sharer identity in social media frameworks matters, and for whom. This paper addresses these limitations in the literature by presenting an original survey experiment, which represents a key component of a broader “big data” project with over 1.8 million registered U.S. voters on the causal effects of social media behavior. The experiment will be conducted as a survey module (n=1,000) within the framework of the Cooperative Election Study’s (CES) November 2021 survey in the United States. The survey experiment will estimate the relative impact of exposure to political content from either the mainstream media, politicians, or one’s social network on support for the vaccination of children against COVID-19. Considering the high proportion of citizens in the United States who use social media for political purposes, this study offers important evidence on what sharer identity may have the most potential to shape public support for salient policies.