But as systems become more aware about the profiles of their visitors - it will become easier to ascertain who the specific individual is that is accessing your site. In some cases, this is already a possibility.

http://www.gabrielweinberg.com/blog/2010/05/a-fb-ad-targeted...

Now imagine a marketplace where Google, Facebook (or somebody that can connect an authenticated user to a digital "fingerprint" of their machine based on browser metrics, installed fonts, ip address, etc.) can answer the "who" question for partner sites willing to buy that information. Visitor-identification as a service.