"Come on. Everybody's doing it," they'll tell you.

These are the same people _ friends, co-workers, relatives _ who earlier twisted your arm to join other networking platforms such as Facebook, MySpace and Friendster.

Dan Costa isn't one of them.

As executive editor for PCMag.com, he's well-versed in social networking technology. He's also a devoted Twitter head.

But he doesn't proselytize. Instead, he wants people who decide to try out services like Twitter to appreciate their power _ for both good and bad.

In a recent interview, Costa warned that many people don't understand that sharing too much personal information can lead to serious consequences. That goes for all of the services, MySpace, Facebook and Twitter, he said.

"The thing that's so unusual about Facebook, is that it does seem so personal," Costa said. "Some of this information you are sharing is so personal. It does make people say, 'Wait a minute. What information is out there?' The answer a lot of times is a lot more than you know about. A lot more."

That painful lesson was learned by a Newport News, Va., police officer whose expletive-laced blog postings got him in hot water last month. On his MySpace page, he wrote about his dismissive feelings about homeless people, complained about the city treasurer and bragged about how the city paid him "to basically drive around and burn their gas while I pick on and harass people who are littler than me."

The officer said he thought his posts were private _ just intended for a circle of friends _ and not widely available on the Internet.

Costa wasn't surprised by this story.

"The rules have changed, and it's not just about social networking either," he said. "Right now, online, anything you do is being recorded. That's the default position you have to take. ... Whether you're blogging or using Twitter or even sending an e-mail, you have to assume that it will live forever. That's a fundamental shift in how we view our privacy."

What's more, social networking sites might be the least dangerous, he said. By their very nature, users can share their complaints and push those who control the sites to make changes.

The social networking services tend to be more open than many other online enterprises, he said. Any time you make a purchase over the Internet, for example, someone is probably compiling information about you.

"The information is moving whether you know it or not," Costa said. "The thing about social networking is it tends to be more public, more out in the open.