Facebook Website Custom Audiences
You can generate cheap leads, turn website visitors into customers, and increase Facebook fans and email subscribers with this Feature. The website custom audience feature gives you a chance to build an audience of people that you know are interested because they've visited your website.
With the website custom audience you'll get a snippet of code to put on your website. You only need to do this once.
The code is pasted to your site and allows you to create a Facebook audience from anyone who has visited your site or certain pages on your site. This works using a browser "cookie" that adds them to the audience you designate the next time they visit Facebook.