The people behind the Chrome browser extension Marauders Map say it's "not an actual tool to creep on your Facebook acquaintances." But that's exactly what it does.
Using geolocation data baked into Facebook Messenger, the extension -- named after a magical document in the "Harry Potter" stories -- plots the locations of people you chat with on a map that pops up over your browser window.
"If you and I were talking on Facebook and we both had location awareness turned on in our settings, this extension can map out where you and I are in the world," CNET's Jeff Bakalar explained.
When someone sends a message, the location from which he or she posted is added to the map, marked by a "bubble" showing the person's profile picture. Clicking the bubble brings up all past location points recorded for that user, accurate to within about three feet.
To use the map, you have to get it from the Chrome web store and open your Facebook messenger conversations (facebook.com/messages). Clicking on a conversation will then load the latest known locations of any participants.
The creators wrote on the website for the extension that they developed Marauders Map as a demonstration of how it is possible to exploit the information collected by social media sites.
So how can you be sure the Marauders Map or some other tracking method won't be used against you in a nefarious way? Bakalar offered some simple free advice:
"You have a cellphone, right? You're walking around with a GPS locator in your pocket," he said. "You can opt out of a lot of that location stuff -- the Instagram stuff, the Facebook stuff. Turn all that junk off. It's unnecessary and you don't need it."