NEW YORK (Fortune) -- Google recently released its annual rundown of popular searches for 2008 - what they call the "zeitgeist" list - and it's a reminder, once again, of how much we reveal about ourselves every time we type into a search bar.

We're interested in Obama, but we're also extremely fond of posing questions to Google like, "what is love?" and "what is life?" (also popular in the "what is" category: "java" and "scientology").

Search engines have become modern society's version of an oracle - the place to ask all your questions, and after some churning of algorithms, something akin to an answer pops out.

Alone with just your computer screen, these searches can feel very private. But Google & Co. gather a lot of information about you as you surf, including the date and time for your search, your search terms, and your IP address, which can help identify your computer and, more important as far as advertisers are concerned, your location. (An IP address, as you'll recall, is made up of 4 sets of numbers, ranging from 0-255, separated by periods. Each number is called an "octet." For example, 22.231.113.67.)

Companies like Google (GOOG, Fortune 500), Yahoo, and Microsoft know it's in their best interests to guard user information very closely. But that information is also key to their business models: they need user data to improve their search engine formulas and, to attract advertisers, match a marketing pitch to an individual's interests.

So if you're a 33-year-old working female who lives in New York City and who likes to search for Jimmy Choo pumps, you might see ads for a local shoe store - thanks to the personal information the search engines have about you.

"There are many free online tools, but they're not really free," explained Greg Conti, a professor of computer science at West Point and the author of Googling Security: How Much Does Google Know About You? "We end up paying for them with micro-payments of personal information which, in turn, are captured and used for data mining and targeted advertising."

Protecting your identity - is it enough?

Still, the practice of gathering and keeping user information has long been controversial. Critics say it violates users' privacy. The search engines argue that user data is crucial to their ability to innovate and stay competitive.

Earlier this month, Yahoo (YHOO, Fortune 500) said it would keep personally-identifiable information in its database for no more than 90 days, down from 13 months previously. This fall Google said it would keep your data for nine months instead of 18 months. Microsoft retains your information for 18 months.

At that time, the companies say they delete or disguise any information that could be tied to a specific individual, including IP addresses, in case the information is accidentally leaked or is stolen. They keep other information like your search terms and the time you conducted a search.

There are big differences, however, in the way that companies make user information anonymous. Google (MSFT, Fortune 500) and Yahoo, for instance, block the last octet of your IP address. Microsoft (MSFT, Fortune 500) says it deletes the entire address.

Opponents, however, say Google and Yahoo's measures don't go far enough. "[Removing the last three digits of the last octet] is like taking the last digit or two off a phone number," argued Marc Rotenberg, executive director of the Electronic Privacy Information Center. "It's the preceding digits that give you the most information."

A cautionary tale

All of the hand-wringing raises an important question: how much do users really care about their privacy?

Conti, the West Point professor, interviewed some 250 college students about two years ago and found that they knew or suspected their surfing habits were being recorded, but said they really didn't care. "They needed the quick fix of information and that outweighed any longer term concerns [about privacy]," said Conti.

This isn't all that surprising in the Facebook era. Many of these same college students are probably avid members of a social network, where they happily divulge their favorite movies, what they did over the weekend, and their relationship status. Marketers, in fact, point out this free flow of information in arguing that people actually want to see advertising tailored specifically for them.

So does privacy really matter? It should.

Consider AOL. In 2006, the Web portal accidentally released details of up to 20 million Web searches by some 650,000 of its users. Names weren't attached to the searches, but they were disturbing in their level of intimacy.

At the site AOL Stalker, which has the leaked search results, the most popular search is User# 672368. In the course of two months, this user searched "curb morning sickness," "you're pregnant he doesn't want the baby," "baby names," "abortion clinics charlotte nc," and "engagement rings" - in that order. (AOL and Fortune.com are owned by Time Warner (TWX, Fortune 500).)

It's not just personal embarrassment that's at stake, argue privacy advocates. Merger talks between two companies, for instance, could be compromised if high-level employees at either company used the Internet to search for information about the potential partner - and those searches, along with the employees names, were made public.

Here's an idea for how to raise consumer awareness: If everyone looked at their histories, we might realize just how much information we're handing over and entrusting to others. Google offers just such a feature, called "web history," and AOL Search has a link on its homepage where users can view their search history from the past 30 days. (Yahoo and Microsoft do not offer this kind of feature.) For many people, looking at past searches would be like opening up a personal diary.