Purdue University researchers studied several popular free apps, including Angry Birds, the New York Times and Free Chess, Call 6 Investigator Rafael Sanchez reported.

They found that the free apps consume a big chunk of energy not to run the game, but to keep the device's GPS on to create customized advertisements.

"That does add up," said professor Charlie Hu.

Of course, the app creators aren't hiding what they're doing. They disclose their intentions before users hit the download button.

Still, the information surprised some Purdue students.

"It's kind of creepy," another woman said.

"No tracking please. I value my privacy," one woman said. "I want to play my game and be left in peace."

Gene Spafford, the director of the Center for Education and Research in Information Assurance and Security, said social media has improved communications and convenience, but that people must be vigilant about what they share online and through mobile devices.

"We should realize it's going to large audience, going into storage we don't know about, government agencies, and other people may be tracking," he said.

There is a movement under way in the social media world to allow users to opt out of having their information shared or to have users pay to keep their information private.

Experts stress that reading the disclosure statements before downloading apps is the best chance to determine how much information will be shared.