If you feel like a shock, try finding out how many online advertising companies are tracking you every time you use the internet.

One way to do so is to go to the Network Advertising Initiative site in the US (www.networkadvertising.org) and click on the opt-out button that allows you to evade their surveillance. It also tells you how many have been watching you already.

My laptop browser, for example, contains cookies (small text files that hold passwords and other data that are used when you browse websites) planted there by 14 advertising networks, such as Google’s DoubleClick, Revenue Science and Tacoda.

I did not know they were there before I looked, although I suspected some were. I gave permission when I signed up for the sites of publishers such as FT.com or WSJ.com and many others. Even using search engines such as Yahoo and Google exposes you or, more precisely, your browser to being trailed.

“The reality is that people have had little choice in terms of online privacy. Try browsing the internet after switching off your cookies and see how well it works,” says Kent Ertugrul, chief executive of Phorm which crunches data obtained from internet service providers.

You can read the rest here and comment below.