A new feature in Facebook's mobile app is a timely reminder of the eavesdropping potential of smartphones.

The feature uses the built-in microphones in mobile devices to determine what music a user is listening to or what television show or movie they are watching. The company unveiled the feature Wednesday and said it would be available for Android and iOS users in the United States in coming weeks.

The feature doesn't operate automatically, and the user must turn it on for it to work, said Facebook product manager Aryeh Selekman. But if a user leaves the feature turned on, it will listen as they use the Facebook app to write status updates, upload photos or respond to messages from friends.

However, "no sound is stored and you’ll always get to choose whether you post to your friends," Selekman said in a blog post.

Facebook said the feature can't identify background noises or conversations. Instead it can only listen and find matching content, similar to some other music match apps already on the market such as Shazam or SoundHound.

The new feature builds on another recent addition to the Facebook platform. Last year, the company added emoticons that symbolize activities or emotions to its status updates. Many industry observers have questions whether that move was meant to make it easier for Facebook to classify user activities and interests for advertisers.

According to Selekman, users have shared more than 5 billion status updates using the emoticons since they were introduced.

While Facebook's new feature sounds rather innocuous in its current form, it also highlights one of the risks of walking around with the equivalent of a high-tech tracking device in your pockets at all time: Someone could be listening in -- whether it be a legitimately installed app, hackers or law enforcement agencies.