In this paper, we introduce DeSo, short for "decentralized social," the first and only blockchain custom-built from the ground up to power and scale a new category of decentralized social applications to one billion users. While much research has been dedicated to scaling "decentralized finance" or "DeFi" applications, relatively little has been invested in building blockchains that can scale social media applications, even though the latter category is arguably just as large, if not larger, and holds just as much promise for value creation. Moreover, while several general-purpose blockchains tout their ability to scale to tens of thousands of transactions per second, scaling up posts per second is a very different problem, and none of these blockchains is currently equipped to handle the storage and indexing requirements of social media applications at scale. To use an analogy from the centralized world, the infrastructure that powers the New York Stock Exchange today is vastly different from that which powers Facebook, Instagram, or Twitter -- they have completely different architectures tailored to support their respective applications at a massive scale. Similarly, our thesis with DeSo is that a blockchain that can scale decentralized social applications to one billion users will likely look vastly different than a blockchain that can scale DeFi apps to the same level. Thus, we believe the future of crypto does not consist of a single general-purpose blockchain that rules them all, but rather a series of dominant, specialized blockchains, each tailored to a particular category of applications. DeSo represents a more than two-year effort to create a blockchain capable of decentralizing the social media category, and we believe it presents the first clear path to solving the existing problems that plague social media today.