Expressing opinions on social media has become a standard form of participation in the political process, but we know little about the factors that shape it. In this paper, we investigate the role of social context. Decades after the development of the canonical "Spiral of Silence" model, the public sphere has radically shifted toward a networked space mediated by social platforms. We articulate a theory of social influence in social media expression and test it by analyzing unique datasets linking U.S. survey respondents to their public Twitter accounts. To measure political expression, we develop and validate a supervised classifier of tweet-level ideology and apply it to respondents' tweets and the tweets of people they follow. We find that the ideology of Twitter followees' tweets is predictive of respondents' own expressed ideology on Twitter, even after holding constant self-reported ideological predispositions. Finally, we present an experimental study in which we directly manipulate Twitter users’ networks, allowing us to measure over-time changes in opinion expression.