As I said when this first came out, this level of targeting is a game changer for a lot of industries.
The world in which marketers had no targeting access to Facebook users’ email addresses is a thing of the past that ended in September 2012 with the advent of “Custom Audiences.” The tool allows marketers to link Facebook marketing to emails, phone numbers, and Facebook UIDs of users who already established a relationship with specific companies off the social network.
via Facebook Once Said No Targeting Users’ Emails – Business Insider.