An extension for the Google Chrome browser can show you where both you and your contacts were located when messaging one another.
It’s so accurate that it can pinpoint past locations to within a metre.
The software – called Marauders Map – was created by student developer and future Facebook intern Aran Khanna.
It’s perfectly legal, and simply uses locations already tied to your Facebook messages to populate a map.
Only conversations where both you and your friends have had location sharing enabled will appear, however.
Once the extension is installed, opening the Facebook messages page will allow you scroll through your conversations and see a map in the bottom left corner of the screen being populated with friends as you go.
Clicking on a person’s chat bubble brings up a detailed spiderweb of you and your friend’s locations throughout your messaging history.
Mr Khanna wrote in a blog post: “By gathering a couple weeks’ worth of chat data on the map and looking at the location clusters you can even figure out (someone’s) weekly schedule.
“With this you can predict exactly which building he would be in at a given time.”
If you have privacy concerns about the public location data, you can turn off the location sharing feature in the Facebook app’s settings.