Facebook’s Instant Personalization: An Analysis of Fundamental Privacy Flaws « 33 Bits of Entropy
Instant Personalization allows a partner website to automatically learn the identity of a visitor (as well as some data about them) without any explicit user action, provided that the visitor is a logged-in Facebook user. It is probably the most privacy-intrusive change introduced by the company this year, and could lead to a profound change in how the web works and is perceived.