If you haven’t yet implemented Custom Web Audiences in Facebook, then you continue to miss out on an amazing marketing opportunity.
And the recent updates (starting to roll out now) are even more useful. Jon Loomer has an excellent hands-on overview of the new CWA enhancements. In particular the ability to create audiences based on the number of times a visitor has been to your site (or a particular URL) in the last X days. Powerful stuff.