In this paper we propose a new class of attacks that exploit advertising systems offering microtargeting capabilities in order to breach user privacy. We study the advertising system offered by the world's largest online social network, Facebook, and the risks that the design of the system poses to the privacy of its users. We propose, describe and provide experimental evidence of several novel approaches to exploiting the advertising system in order to obtain private user information. We communicated our findings to Facebook on July 13, 2010, and received a very prompt response. On July 20, 2010, Facebook launched a change to their advertising system that made the kind of attacks we describe much more difficult but not impossible to implement.