Every time cellphone users check in to their favorite place on social media, post pictures of their food, track their workouts, or simply browse the Internet, they are giving personal information to phone carriers. Last week, AT&T reminded us of this fact when it quietly updated its privacy policy to include details about how it will begin to sell such data to the highest bidder.

AT&T will be selling such customer information as location based on Wi-Fi networks, Web browsing data and mobile app usage, all in the name of providing targeted ads that are theoretically more relevant than those that don’t incorporate such specifics.

Although AT&T is simply joining other companies such as Google, Facebook, Verizon and T-Mobile that have been collecting data for some time now, its position as the second-largest wireless carrier (105 million subscribers) makes the company’s announcement worth noting.

If there’s any good news to be had, it’s that AT&T promises the data will be anonymous and aggregated, meaning customer names and other identifying details will not be included. To opt out of the external marketing data share, users can go to this AT&T page. Keep in mind that customers must opt out from each computer browser or wireless device that they wish to exclude.

Read the letter AT&T sent to its customers last week here.

–Posted by Rosa Trieu