One night last summer she noticed on Loopt that friends she was meeting for dinner were 40 miles away, and would be late. Instead of waiting, Ms. Fong arranged her schedule to arrive when they did. “People don’t have to ask ‘Where are you?’ she said.

Image A phone with Buddy Beacon, a tracking service offered by Helio, a mobile phone service provider.

Ms. Fong can control whom she shares the service with, and if at any point she wants privacy, Ms. 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 where I was all the time,” she said.

Some situations are not so clear-cut. What if a spouse wants some time alone and turns off the service? Why on earth, their better half may ask, are they doing that?

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.

But 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, which follows trends in technology.

So far, it is most popular, industry executives say, among the college set.

But others have found different uses. Mr. Altman said one customer bought it to keep track of a parent with Alzheimer’s. Helio, a mobile phone service provider that offers Buddy Beacon, said some small-business owners use it to track employees.