With the advent and unprecedented popularity of the now ubiquitous social networking sites such as Google Friend, Facebook, MySpace, Twitter etc. in the personal sphere, and others such as LinkedIn in business circles, undesirable security and privacy risk issues have come to the forefront as a result of this extraordinary rapid growth. The most salient issues are mainly lack of trustworthiness; namely, those of security and privacy. We will address these issues by employing a quantitative approach to assess security and privacy risks for social networks already under pressure by users and policymakers for breaches in both quality and sustainability; and will also demonstrate, using a cost-optimal game-theoretical solution, how to manage and monitor risk. The applicability of this research to diverse fields from security to privacy and health care, as well as the currently popular social networks is an additional asset. A number of real people (not simulated) were interviewed and the results are discussed. Ramifications of this quantitative risk assessment of privacy and security breaches in social networks will be summarized. (C) 2012 Published by Elsevier Ltd.