This paper proposes a new normative framework to think about Big Tech reform. Focusing on the case of digital communication, I argue that rethinking the corporate purpose of social media companies is a distinctive entry point to the debate on how to render the powers of tech corporations democratically legitimate. I contend that we need to strive for a reform that redefines the corporate purpose of social media companies. In this view, their purpose should be to create and maintain a free, egalitarian, and democratic public sphere rather than profit seeking. This political reform democratically contains corporate power in two ways: first, the legally enforceable fiduciary duties of corporate boards are reconceptualized in relation to democratic purposes rather than shareholder interests. Second, corporate governance structures should be redesigned to ensure that the abstract purpose is realized through representatives whose incentives align with the existence of a democratic public sphere. My argument complements radical proposals such as platform socialism by drawing a connection between democratizing social media governance and identifying the proper purpose of social media companies.