If you were to browse through my computer, looking at my Amazon and eBay searches, you would quickly find that I like classical music, 3D movies, tribute albums that cover the Beatles’ songs, and little toy figures of Mickey Mouse as either Steamboat Willy or The Sorcerer’s Apprentice. And if you browse through my Facebook postings, you’ll see that I like dark chocolate, “Dexter” (the TV series) and elephants.

This is most of what Google, Amazon and Facebook can tell you about me. They won’t, because they say they’re committed to the privacy of their users, but this is what they know about me and this is why Amazon shows me ads for the 3D Blu-ray release of “Dial M For Murder,” eBay tells me that I might be interested in bidding on Joshua Rifkin’s “Baroque Beatles Book” on CD, and Facebook tells me about various matchmaking services—and the occasional underwear ad as well. (I haven’t figured that last one out.)

I don’t normally click on Google’s ads, perhaps three times in the past three years, and only once have I made a purchase from a Gmail ad. I estimate that Google pushes at least 10,000 ads a year to me through Gmail and Google search. If I buy only one item out of 10,000 offers, that’s not exactly cost-effective for Google.

But all those ads are automatically generated, based on the content of my searches and e-mails, and cost Google almost nothing to send. Multiply that by a billion users worldwide and the return rate on those ads will justify the cost. One in 10,000 becomes a hundred thousand click-throughs out of every billion pushes. If even 1% of those click-throughs result in a purchase, that’s a thousand sales. If your product sells for $100, you’ve grossed a hundred thousand dollars.

I suspect that Amazon generates a much higher rate of return because the person came to the site looking for a specific item, already in a browsing and buying mood. Amazon keeps a record of previous purchases and offers suggestions based on that information.

And as Amazon expands their product line, they will gather ever more detailed information about individual customers’ tastes and habits. This summer, when we had a serious ant infestation in our kitchen, we couldn’t find Boric Acid in any local stores—but Amazon had it. I had a hankering for Malt-O-Meal; my local supermarket didn’t have it—but Amazon did. None of the local stores had Rocky Road candy bars—but Amazon did. So now Amazon has become my retailer of choice. In return, Amazon probably knows more about me than the FBI.

The cost of this convenience is my privacy.

This is true for just about everyone who uses online services. If you use the Internet, if you use Google or Bing, if you use Amazon or eBay, if you visit Yahoo or Facebook, if you are active at all online, then you have knowingly given major corporations access to your likes and dislikes.

It goes beyond that. It’s like demographics on steroids. Add in your age, the congressional district you live in, the real estate values of your neighborhood, your proximity to highways, schools, libraries, shopping outlets and entertainment venues, and the first rough outlines of your identity are drawn.

Add some sophisticated data mining and even more detailed demographic profiling, and the software can begin to make some eerily accurate predictions of your behavior—extrapolating purchasing decisions, political views, medical issues, even sexual orientation. Facebook reveals a lot about you simply by whom you have friended; one young man was inadvertently outed when his parents saw that Facebook was sending him gay-dating ads. Neither he nor they were ready to deal with that.

Remember, your entire online history is available for anyone with the tools to go spelunking. Everything. And privacy advocates are justifiably concerned about how much of our personal lives can be gleaned from our online histories. Sophisticated data mining can create a detailed profile of an individual’s behavior far beyond the dreams of J. Edgar Hoover’s ghost.

We have stumbled into this state of affairs because it is convenient. We want our systems to serve us and we’re willing to have them know things about us so they can serve us better. We don’t mind Amazon knowing what we like.

Demographic profiling and cross-correlation is useful to both sides of the buyer/seller equation. It makes it possible for marketers to target their advertising far more effectively and for users to find the products they want.

But more than useful, more than convenient, sometimes it’s life-saving. We want the pharmacist to know what medications we’re taking so we don’t inadvertently experience interactions and side effects. We want the online supermarket to know that the kids have a peanut allergy. We want the fire department to know that there are two dogs in the house.

So we’re not just redefining privacy, we’re creating a new normal. At some point, the legislatures and the courts are going to have to determine what boundaries must be set and where the lines should be drawn. That’s going to affect every company that gathers data from the users of its software. A large part of that is going to be about the security of the data that the software gathers and transmits, where and how it is stored, and who can access it and why. We’re already seeing how dangerous a breach can be, but pay attention—this one has the potential to get very ugly. Uglier than it already is.

What do you think?

David Gerrold is the author of over 50 books, several hundred articles and columns, and over a dozen television episodes, including the famous “Star Trek” episode, “The Trouble with Tribbles.” He is also an authority on computer software and programming, and takes a broad view of the evolution of advanced technologies. Readers may remember Gerrold from the Computer Language Magazine forum on CompuServe, where he was a frequent and prolific contributor in the 1990s.