What is Facebook Connect?
A social plug-in tool that connects your Web activities with your Facebook activities.
How does Facebook make the connection?
Every Facebook profile has an ID or a username you selected. You profile ID is a key that is used to uniquely identify you to Facebook. Your profile ID or your username will appear in the URL if you click on your profile.
If you visit a page that has Facebook Connect (for example, CNN.com), then Facebook connect will log your visit by sending a message with your profile ID.
The image below shows messages to Facebook that include my profile ID (54902736). If you are logged out of Facebook, the message is still sent. As long as there is a Facebook cookie on your machine, your profile ID will be logged.