When an internet user visits any website containing a Google ad....

Such as DU, they probably get tracked and subjected to custom adverts, where ever you go.

There you are:
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A deeper peeper

By Maija Palmer

Published: April 15 2009 03:00 | Last updated: April 15 2009 03:00

Twenty-five years on from the spectre evoked in 1984 , the extent to which big governmental and corporate brother is watching you is biggest of all online. Anyone who Googles for a website, looks up a friend on Facebook or browses the books on Amazon.com is likely to have those actions recorded and stored on a database somewhere.
~snip

When an internet user visits any website containing a Google ad, the visit will be recorded and used to compile a profile that will then be deployed to direct future advertising to the user. Anyone who, for example, surfs a number of sites related to do-ityourself projects will be noted as a home improvement fan and will begin to see advertising for paint and power tools turn up on all manner of sites.
~snip


Sites such as Facebook and Flickr store billions of photos of people. Facebook says more than 1bn are uploaded every month. Face.com, an Israeli company, is launching facial recognition software that can sift through these better. Internet-connected mobile handsets such as the iPhone are capable of tracing someone's location at any time.

Collecting information is itself hardly new but in the past it has been labour-intensive and limited. It has never before been possible to collect so much information about so many people and use it so easily. "There has been a transfer of control over our personal information," says Mr Rotenberg of Epic. "Individuals are not easily able to control the information or opt out, which means these issues need to be raised as broader public policy issues or in the courts." Epic recently filed a complaint to the US Federal Trade Commission over Google's "cloud" computing service, which provides storage and programming capacity on the company's servers.

http://www.ft.com/cms/s/0/f9682b48-2953-11de-bc5e-00144feabdc0.html?nclick_check=1

The benefits substnatially outweigh the costs to me as an individual.