Social networking sites (SNSs) allow individuals to establish and maintain a variety of relationships as well as share different aspects of their identity by expressing their views on numerous topics, including politics. SNS also come with perceived interpersonal risks and benefits tied to sharing with a collapsed networked audience. Using a nationally representative sample of US social media users ( N = 2,873) from 2016, this study investigated how perceived network characteristics influence people’s decision to engage in online political expression on three platforms: Facebook, Twitter, and Instagram. Findings indicate that perceived ideological homophily with the audience on an SNS and past use of privacy management settings both predict how much individuals post about politics on Facebook and Twitter, but not on Instagram. On Instagram, Black Americans were significantly more likely to engage in online political expression. On Facebook and Twitter, older Americans engaged in more political expression, and across all platforms, perceptions that political discussion online is uncivil were negatively associated with political expression.