Here's some technical notes about how facebook is tracking. Looks like they're using images to do it.
This is a general overview:<br/>
+ <br/>
+ <img src="etags.jpg"/><br/>
+ <br/>
+ The ETag shown in the image is a sort of checksum. When the image changes, the checksum changes. So when the browser
+ has the image and knows the checksum, it can send it to the webserver for verification. The webserver then checks
+ whether the image has changed. If it hasn't, the image does not need to be retransmitted and lots of data is saved.<br/>
+ <br/>
+ Attentive readers might have noticed already how you can use this to track people: the browser sends the information
+ to the server which it just received. That sounds an awful lot like cookies, doesn't it? The server can simply give
+ each browser an unique ETag, and when they connect again it can look it up in its database.<br/>
+ <br/>
+ And that's what this page does too.<br/>
+ <br/>
+ <b>Technical stuff</b> (and bugs in this demo)<br/>
+ For demonstrational purposes I want to show you what I store without having to use Javascript, which creates some
+ restrictions on what I can do. Because the page is loaded before the hidden image is loaded (ETags on pages do not
+ work very well, you need to use an image), and I want to show the data in the page, we have a chicken and egg problem.
+ To solve this I use your IP address as the common piece of information, but this would not normally be needed.
+ Not that trackers won't use it, your IP is a great method of identification even when you use a proxy, but it's
+ just not required for this technique.<br/>