Contemporary liberal accounts of free expression are almost exclusively preoccupied with the permissible exercises of state power. Influenced by this framing, free expression guarantees under the ECHR, as well as the US and German Constitutions, focus on protecting a private sphere from state interference: what happens within that sphere is only of peripheral concern. This approach is deeply unsatisfactory, especially given the significant threats emanating from private social media platforms that shape the conditions under which individuals may express themselves online. The article argues that we should take private platforms seriously as a source of significant threats, without abandoning the distinction between private actors and the state. Private platforms that are generally open to the public should have obligations to uphold free speech in their contractual relationship to users under certain conditions: if they are structurally dominant, make arbitrary decisions or significantly impact a user's societal participation.