Two new questions arise, courtesy of the latest advancement in cell phone technology: Do you want your friends, family, or colleagues to know where you are at any given time? And do you want to know where they are?

Obvious benefits come to mind. With new services like Sprint Nextel's Loopt that take advantage of the Global Positioning System chips embedded in many cell phones, parents can track the whereabouts of their phone-toting children.

And for teens and twentysomethings, who are fond of sharing their comings and goings on the Internet, such services are a natural next step.

Sam Altman, the 22-year-old co-founder of Loopt, said he came up with the idea in early 2005 when he walked out of a lecture hall at Stanford University.

"Two hundred students all pulled out the cell phones, called someone and said, 'Where are you?"' he said. "People want to connect."

But such services point to a new truth of modern life: If GPS made it harder to get lost, new cell phone services are now making it harder to hide.

"There are massive changes going on in society, particularly among young people who feel comfortable sharing information in a digital society," said Kevin Bankston, a staff lawyer at the Electronic Frontier Foundation based in San Francisco.

"We seem to be getting into a period where people are closely watching each other," he said. "There are privacy risks we haven't begun to grapple with."

But the practical applications outweigh the worries for some converts.

Kyna Fong, a 24-year-old Stanford University graduate student, uses Loopt. For $2.99 a month, she can see the location of friends who also have the service, represented by dots on a map on her phone, with labels identifying their names. They can also see where she is.

Fong can control with whom she shares the service, and if at any point she wants privacy, Fong can block access. Some people are not invited to join — like her mother. "I don't know if I'd want my mom knowing were I was all the time," she said.

Users can also turn off their service, making them invisible to people in their social-mapping network. But there are downsides to that as well. What if a spouse's phone goes dark? Why on earth, their better half may ask, are they doing that?

Or what if a boss asks an employee to use the service?

So far, the market for social-mapping is nascent — users number in the hundreds of thousands, industry experts estimate.

Still, almost 55 percent of all mobile phones sold today in the United States have the technology that makes such friend and family tracking services possible, according to Current Analysis.