Virtually everything you do online is scrutinized by search engines and advertising networks that evaluate you as a potential customer based on what you search for, the sites you visit and the ads you see -- whether you click on those ads or not.

"It's as though every time you pick up a magazine or a book or you browse a storefront, you might be reading the magazine, but it's reading you back, and the ads in the magazine are reading you," said Peter Eckersley, senior staff technologist for the Electronic Frontier Foundation, a civil liberties organization that monitors the online world.

Marketers argue that "behavioral advertising" -- which serves up ads based

on a person's browsing history and demographics -- is good because it produces ads that fit a person's interests. But privacy advocates like Eckersley say the "ubiquitous surveillance" violates "a fundamental civil liberty" -- the right to read in private. Another threat, he said, is that someone else could get hold of your data.

So if everything on the Web has eyes, how do you draw the shades?..