What’d they do this time? Oh, install some cookies that track your entire browsing history. Well, sort of.
The problem was, and actually still is, this: Facebook has several cookies that report your Facebook ID back to the site or can otherwise be used to track you, even if you log out. In and of itself, this isn’t much of a web tracking device, really — mostly only sites that use Facebook Connect would be sending cookies back to Facebook, which means the site could be logging what you do.
Wait, you mean millions of sites use Facebook Connect? Uh-oh.
The most problematic one, “a_user,” is now shut down on logout, although this does require you to actually log out of Facebook, something not a lot of people do on their private computers. The responsible thing, of course, would be to terminate the cookie completely, but the Zuckerbeast must be fed that sweet, sweet information.