But the use of "geosocial" services to "check in" to certain locations or share one's location with friends dropped from 18 percent in early 2012 to 12 percent a year later. And as of September 2012, 46 percent of teen app users reported that they had turned off the location-tracking feature in their device or in an app on that device.
Privacy experts say all mobile users should disable location tracking unless they are actively using an app, like a map program giving them directions. "I generally disable location services except for specific apps at specific moments, such as I'm trying to use Google maps to find a specific place," said Hanni M. Fakhoury, a staff attorney at the Electronic Frontier Foundation (EFF).
They also advise strongly against allowing social media posts to include location. It's not just the obvious risks of publishing the fact that you may be far from home, offering an invitation to burglars. It is also the cumulative impact of thousands of little details about your associations, your beliefs, your habits — your life.
"Mobile apps are of special concern because smartphones tend to get exceptional data about us — what time you wake up, when you go to the doctor's office and when you go to McDonald's, whether you drive above the speed limit, and on and on," said Ben Edelman, a privacy expert and an associate professor at Harvard Business School.
"Individually, this data might seem unimportant. But add it up — millions of users, over months and years — and it's a portrait of humanity. Never before has so much data been collected about so many. And to what end?"
Rebecca Herold, CEO of The Privacy Professor, warns that, "whenever data is posted online, such as through the auto-location-sharing apps, that data is subject to a wide range of surveillance."
Hanni Fakhoury agrees. "Detailed information about a person's location reveals a lot about that person's associations and activities. And law enforcement is eager to get its hands on that information," he said.
The companies that make the apps and provide the services that use location tracking generally make a point of promising that the user has control over what is shared. Google recently amended its Terms of Service to include what it calls a "shared endorsements" setting that, starting Nov. 11, will show Google+ users' — including their profile picture — product preferences alongside ads within their social network.
Google's pitch is that the new setting will, "make it easy for you to get great recommendations from your friends." But it emphasizes that, "You're in control: Your content is only shared when you choose, and shared."
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