Tracking Personal Identifiers Across the Web

User tracking has become de facto practice of the Web, however, our understanding of the scale and nature of this practice remains rudimentary. In this paper, we explore the connections amongst all parties of the Web, especially focusing on how trackers share user IDs. Using data collected from both browsing histories of 129 users and active experiments, we identify user-specific IDs that we suspect are used to track users. We find a significant amount of ID-sharing practices across different organisations providing various service categories. Our observations reveal that ID-sharing happens in a large scale regardless of the user profile size and profile condition such as logged-in and logged-out. We unexpectedly observe a higher number of ID-sharing domains when user is logged-out. We believe that our work reveals the huge gap between what is known about user tracking and what is done by this complex and important ecosystem.