Social media are changing contemporary practices of communication, information, and participation. They continue the tradition of “Web 2.0”—a set of interconnected changes in software architecture, business models, and guiding principles that transformed online communication in the early 2000s. Two fundamental innovations characterize the current form of social media: first, a communication architecture based on personalization and the convergence of conversation and publication; second, a business model grounded in the comprehensive datafication of user activities. Both aspects contribute to a profound transformation of the public sphere—making it more open to diverse and alternative voices, but also more vulnerable to extremist communication and disinformation. The unprecedented economic power of a few dominant platforms in shaping these structures poses a threat to democratic models and ideals of public communication. Far from the utopian discourses of the participatory web, social media give rise instead to paradoxes, conflicts, and contradictions that complicate the establishment and further development of these media innovations.