Facebook promised users privacy, but gave them surveillance. This is detailed in the paper The Antitrust Case Against Facebook (backup draft version here):
Abstract of the paper:
The Facebook, Inc. ("Facebook") social network, this era's new communications service, plays an important role in the lives of 2+ billion people across the world. Though the market was highly competitive in the beginning, it has since consolidated in Facebook's favor. Today, using Facebook means to accept a product linked to broad-scale commercial surveillance - a paradox in a democracy. This Paper argues that Facebook's ability to extract this qualitative exchange from consumers is merely this titan's form of monopoly rents. The history of early competition, Facebook's market entry, and Facebook's subsequent rise tells the story of Facebook's monopoly power. However, the history which elucidates this firm's dominance also presents a story of anticompetitive conduct. Facebook's pattern of false statements and misleading conduct induced consumers to trust and choose Facebook, to the detriment of market competitors and consumers' own welfare.
If antitrust law isn't up to the job, consideration should be given to updating antitrust law for the digital age in which products frequently appear to be priced at zero, but monopolies still form due to strong network externalities.