I think that what makes this case particularly special is that Facebook referring URLs share much more data about you than the average site. A typical Facebook URL can be something like:

http;//facebook.com/#!profile_id=123/reqs.php/456/v=photos&ref=pymk

This means "I am user 123 and I'm looking at the photos of user 456 after having clicked through to their profile. I found this user's profile through Facebook's friend recommendation page."

Why does Facebook have to put all that info in the URL in the first place?

The referring URL for an average site would simply share "I am an anonymous user that's looking at 456's photos".

An advertiser could use Facebook's Graph (where your name, picture and other information is forced to be public now and indexed via the above Ids) and you have extremely detailed info about someone and their Facebook activity.

Note: It looks like Facebook has stripped the part of the URL that needlessly self-identifies now, so that's good.