This dissertation explores what it means for writers to maintain agency and ownership over their textual productions in big data age, where to write means to participate in a complex weave of software, code, and networked algorithms, and where writing produces both conventional text and data. Given how much everyday writing flows through proprietary digital platforms, my dissertation asks: how can we carve out a model of ownership that centers the agency of writers and users in the face of corporate web platforms that aggressively appropriate the value of our textual productions? Digital writing scholarship has responded by appealing to copyright law and proprietary authorship; however, calls to property or authorship no longer work towards the advantage of writers because they are rooted in an industrial capitalism framework where the propertization of writing was primarily expressed through copyright. Attending closely to Web 2.0’s infrastructure, I show that property on the digital web has shifted from texts (copyright) to platforms (the means of producing texts), and that the function of property has shifted from rewarding authors to appropriating the value of their labor. Drawing on classical and contemporary political economic theory, I develop a framework for writer agency grounded in a critical theory of property, where agency is tied to the struggle against the propertization of writing and writing technologies and with writer control over the circulation infrastructures that make up the web. I test this theoretical model of agency against existing attempts to create alternative writing platforms that center writer agency, and by looking at the experiences of writers on those platforms. My findings show, first, that the most successful alternative platforms—free and open-source platforms like Mastodon—attempt to empower writers through decentralized server structures that make large-scale data appropriation and production impossible. Second, I show that solving the issue of data appropriation is not enough; writers need control over the governance and design of a platform in order to attain agency in Web 2.0. However, existing platform models, including Mastodon, do not yet achieve this standard. Together, these findings suggest that digital writers need more ambitious writing instruction that treats server architecture, platform and interface design, circulation protocol and content moderation, and platform governance as part of what it means to write in Web 2.0.