The Twitter Abacus: Silently Logging Every Link You Click On (twitter.com/abacus)

This afternoon, I was using the HttpFox Firefox extension to analyze some web-traffic for a work related project.  With HttpFox still running in the background (I forgot I left it running), I opened another tab and navigated over to my Twitter page to check out a few things.  I clicked a few links, replied to a few folks, etc.  Switching back to my work project, I closed Twitter and re-opened HttpFox.  Well well well, what do we have here.  I discovered that Twitter silently rolled out some JavaScript that actively tracks every link I click on.  Any link, in any Tweet, that you click on is silently reported back to Twitter behind the scenes.  Looking at the output of HttpFox pretty much proves it:



Looks like twitter.com/abacus is some type of web-service used by Twitter to log what links we're all clicking on.  I'm curious why Twitter cares what links we're clicking on.

BTW, Twitter, if you're reading this, the HTTP Content-Type in your responses from /abacus are incorrect.  You're phoning home by creating a new Image() in your core JavaScript like so:

(new Image()).src="/abacus?"+$.param(A);

But your Content-Type from this request is text/html, which could cause problems in a few browsers.  If you're going to use an Image(), the returned Content-Type from your /abacus web-service should be that of an image: image/jpeg, image/png, image/gif, etc.