Social networks have been studied for a century  and are a staple of research in disciplines such as epidemiology , sociology [82; 32; 12], economics , and many others [22; 9; 36]. The recent proliferation of online social networks such as MySpace, Facebook, Twitter, and so on has attracted attention of computer scientists, as well .
Even in the few online networks that are completely open, there is a disconnect between users' willingness to share information and their reaction to unintended parties viewing or using this information . Most operators thus provide at least some privacy controls. Many online and virtually all offline networks (e.g., telephone calls, email and instant messages, etc.) restrict access to the information about individual members and their relationships.
Network owners often share this information with advertising partners and other third parties. Such sharing is the foundation of the business case for many online social-network operators. Some networks are even published for research purposes. To alleviate privacy concerns, the networks are anonymized, i.e., names and demographic information associated with individual nodes are suppressed. Such suppression is often misinterpreted as removal of "personally identifiable information" (PII), even though PII may include much more than names and identifiers (see the discussion in Appendix B). For example, the EU privacy directive defines "personal data" as "any information relating to an identified or identifiable natural person [...]; an identifiable person is one who can be identified, directly or indirectly, in particular by reference to an identification number or to one or more factors specific to his physical, physiological, mental, economic, cultural or social identity" .
Anonymity has been unquestioningly interpreted as equivalent to privacy in several high-profile cases of data sharing. After a New York court ruling ordering Google to hand over viewing data of over 100 million YouTube users to Viacom and the subsequent protests from privacy advocates, a revised agreement was struck under which Google would anonymize the data before handing it over . The CEO of NebuAd, a U.S. company that offers targeted advertising based on browsing histories gathered from ISPs, dismissed privacy concerns by saying that "We don't have any raw data on the identifiable individual. Everything is anonymous" . Phorm, a U.K. company with a similar business model, aims to collect the data on Web-surfing habits of 70% of British broadband users; the only privacy protection is that user identities are mapped to random identifiers . In social networks, too, user anonymity has been used as the answer to all privacy concerns (see Section 2).
Our contributions. This is the first paper to demonstrate feasibility of large-scale, passive de-anonymization of real-world social networks.
First, we survey the current state of data sharing in social networks, the intended purpose of each type of sharing, the resulting privacy risks, and the wide availability of auxiliary information which can aid the attacker in de-anonymization.
Second, we formally define privacy in social networks and relate it to node anonymity. We identify several categories of attacks, differentiated by attackers' resources and auxiliary information. We also give a methodology for measuring the extent of privacy breaches in social networks, which is an interesting problem in its own right.
Third, we develop a generic re-identification algorithm for anonymized social networks. The algorithm uses only the network structure, does not make any a priori assumptions about membership overlap between multiple networks, and defeats all known defenses.
Fourth, we give a concrete demonstration of how our de-anonymization algorithm works by applying it to Flickr and Twitter, two large, real-world online social networks. We show that a third of the users who are verifiable members of both Flickr and Twitter1 can be recognized in the completely anonymous Twitter graph with only 12% error rate, even though the overlap in the relationships for these members is less than 15%!
Sharing of anonymized social-network data is widespread and the auxiliary information needed for our attack is commonly available. We argue that our work calls for a substantial re-evaluation of business practices surrounding the sharing of social-network data.