In this paper, we examine web user tracking capabilities of the three major global Online Social Networks (OSNs). We study the mechanisms which enable these services to persistently and accurately follow users web activity, and evaluate to which extent this phenomena is spread across the web. Through a study of the top 10K websites, our findings indicate that OSN tracking is diffused among almost all website categories, independently from the content and from the audience. We also evaluate the tracking capabilities in practice and demonstrate by analyzing a real traffic traces that OSNs can reconstruct a significant portion of users web profile and browsing history. We finally provide insights into the relation between the browsing history characteristics and the OSN tracking potential, highlighting the high risk properties.