“David, you’re sounding like an old dude!” Matt Flannery, who runs social-lending website Kiva, couldn’t understand when I explained that, no, I wouldn’t be keeping in touch with him via Facebook. “What are you worried about?” he teased in a break at the PINC conference in Holland. “Only old guys get worked up about privacy.”

Well, Matt, I admit I’m the wrong side of 30, and that I still avoid using emoticons in formal correspondence. But let me explain why I’m not active on Facebook, nor sharing my credit-card purchases on Blippy, nor allowing Google Buzz to mine my contacts list, nor even publishing my DNA on 23andMe.com. My cautious use of the social networks has nothing to do with paranoia about privacy; and yes, I celebrate the unprecedented transparency and connectivity that these services can empower. But what’s increasingly bothering me is the wider social and political cost of our ever-greater enmeshment in these proprietary networks. Here are half a dozen reasons why.

1) Private companies aren’t motivated by your best interests
Facebook and Google exist to make money, by selling advertisers the means to target you with ever greater precision. That explains the endless series of “privacy” headlines, as these unregulated businesses push boundaries to make it easier for paying third parties to access your likes, interests, photos, social connections and purchasing intentions. That’s why Facebook has made it harder for users to understand exactly what they’re giving away — why, for instance, its privacy policy has grown from 1,004 words in 2005 to 5,830 words today (by comparison, as the New York Times has pointed out, the U.S. Constitution is 4,543). Founder Mark Zuckerberg once joked dismissively about the “dumb fucks” who “trust me”. I admire the business Zuckerberg’s built; but I don’t trust him.

2) They make it harder to reinvent yourself
“When you’re young, you make mistakes and you do some stupid stuff,” President Obama warned high-school students in Virginia last September. “Be careful about what you post on Facebook, because in the YouTube age whatever you do will be pulled up later somewhere in your life.” He’s right: anything posted online might come to haunt you permanently, yet all of us need space to grow. As the writer Jaron Lanier said in a recent lecture, if Robert Zimmerman, of small-town Hibbing, Minnesota, had had a Facebook profile, could he really have re-created himself as the New York beatnik Bob Dylan

3) Information you supply for one purpose will invariably be used for another …
Phone up to buy a pizza, and the order-taker’s computer gives her access to your voting record, employment history, library loans — all “just wired into the system” for your convenience. She’ll suggest a tofu pizza as she knows about your 42-inch waist, she’ll add a delivery surcharge because a nearby robbery yesterday puts you in “an orange zone” — and she’ll be on her guard because you’ve checked out the library book Dealing With Depression. This is where the American Council for Civil Liberties sees consumerism going — watch its pizza video online — and it’s not to hard to believe. Already surveys suggest that 35 percent of firms are rejecting applicants because of information found on social networks. What makes you think you can control what happens to your personal data?

4) … and there’s a good chance it will be used against you
Mark Zuckerberg would like to suggest that, in an ever more transparent world, “you have one identity — the days of you having a different image for your work friends or co-workers and for the other people you know are probably coming to an end pretty quickly.” That suits his purpose — but in our multi-layered lives it’s just not true. A vindictive ex-partner, or a workplace rival, or a health insurer, or a political opponent, may selectively expose information to your detriment – powerfully re-framing your identity in a way you would consider dishonest.

5) People screw up, and give away more than they realise
To understand how much personal information Facebook users are inadvertently sharing, visit youropenbook.org and search for phrases such as “cheated on my wife” or “my new mobile number is” or “feeling horny“. I’ll bet that most of the people whose intimate details you’ll get to read are unaware that their updates are being shared quite so openly. Have they genuinely given Zuckerberg their informed consent?

6) And besides, why should we let businesses privatize our social discourse?
Some day you should take time to read those 5,830 words: it’s Facebook that owns the rights to do as it pleases with your data, and to sell access to it to whoever is willing to pay. Yes, it’s free to join — but with half a billion of us now using it to connect, it’s worth asking ourselves how far this “social utility” (its own term) is really acting in the best interests of society.

Don’t take my word, Matt — young internet users themselves are increasingly wary of the social networks’ use of their private data. A recent study by the Pew Internet and American Life Project — a decent sample of some 2,253 Americans — found that 44 percent of Generation Y (aged 18 to 29) now limit their online personal information, compared with 33 percent of internet users between ages 30 to 49. And three-quarters of younger social-networkers have adjusted their privacy settings to limit what they share.

Call me uncool — but that’s a trend I’m happy to share with my friends. In person.