A report in the Wall Street Journal this evening reveals that Facebook, MySpace, Twitter, and a number of other popular social sites are passing along data that advertisers could potentially use to identify users who click their ads.

The article is focused on Facebook in particular, which appears to have been passing along the most data of the aforementioned sites and has also been embroiled in a major privacy controversy The Journal article doesn't get into too much technical detail, but it sounds like Facebook and the others are failing to scrub 'referring' URLs that are always passed along whenever a user clicks a link.

This is actually normal behavior — typically when you click a link on a website, the site you're being directed to will get to see where you came from The issue is that these social sites include some identifying information as part of their URLs; when you visit a friend's Facebook profile, the resulting URL might include both your friend's username and your Facebook ID, which could be used to associate you with the ads you're clicking on.