Network analysis of third party tracking: User exposure to tracking cookies through search

Internet advertisers reach millions of customers through practices that real time tracking of users' online activities. The tracking is conducted by third party ad services engaged by the Web sites to facilitate marketing campaigns. Previous research has investigated tracking practices and tracking agencies associated with popular Web sites. Here we investigate the network properties of the third party referral structures that facilitate gathering of user information for the delivery of personalized ads. By considering third party domains associated with the top ten search results for a diverse set of queries, we arrived at the networks of third party domains in four search markets. We show a consistent network structure across markets, with a dominant connected component that, on average, includes 92.8% of network vertices and 99.8% of the connecting edges. There is 99.5% chance that a user will become tracked by all top 10 trackers within 30 clicks on search results. Finally, the third party networks exhibit properties of the small world networks. This implies a high-level global and local efficiency in spreading the user information and delivering targeted ads.