Marketing insider Mark Johnson says your data trail is worth big bucks to cellphone companies.
Many people have no idea this information is being collected, packaged with details about your age and gender, aggregated and sometimes sold to third parties.
"They know you were playing Angry Birds, they know that you drove by Sears, they know you drove by Dominos pizza, so they can take that and take a very unique algorithm that can focus on your behavior," said Johnson, president and CEO of Loyalty Marketers Association.
Smartphone user Harrine Freeman is so creeped out, she turns off her GPS when she drives and shops. She also clears her browser history.
"I think it's an invasion of privacy," she said. "I don't think cellphone companies should sell your information."
All the major cellphone carriers admit to collecting information.
Verizon acknowledges it aggregates the information and sells it to businesses without personally identifying users.
Some cellular companies say there's an advantage to tracking users. They say users get ads that are relevant and can save them money.
"This is something that consumers are automatically opted into," said Rainey Reitman of the Electronic Frontier Foundation.
Critics say cellphone companies tell customers what data they're collecting by sending them privacy notices that may be difficult to understand and written in fine print. They don't like that consumers who don't want to be tracked have to make the extra effort to opt out.
"I don't really think that most people are going to review every email they get form their cellphone company and then go through the extra step of opting out of this targeted advertisement," Reitman said.
Johnson says be sure to read any updates your carrier sends, too, because tracking technology keeps changing.
"The amount of data these cellphone companies have has grown tremendously over the last three to four years," Johnson said. "With the rapid rise and proliferation of cellphones, it will only continue to grow."
Privacy experts say also be careful of third party apps you download that request to use your location. If you don't want to be tracked always, press no, or don't allow.