Developed by Harvard student Aran Khanna, Marauder's Map pulls data from Facebook Messenger - data that we don't even know is available to the public - to pinpoint the exact location of any of your Facebook friends. The map is able to do this because Facebook Messenger automatically attaches your phone's location to each message you send without you even knowing it.
The Washington Post explains, "Facebook's tracking your longitude and latitude with five decimal places in precision, which means you could theoretically not only know someone's address, but which exact room they were in."
Needless to say, it's creepy, it's a clear violation of privacy, most of us aren't even aware to what extent Facebook is stalking us, and the worst part is Facebook has no intention to fix this security issue.
With the amount of access we have to data-sharing, our privacy could only continue to be in jeopardy. And it's really not our fault. Default settings are deliberately set in this manner by app-creators so that it's possible to track all of our personal information - from what websites you frequently visit to what you are doing throughout the day.
There really is no way to solve the problem other than to carefully monitor the privacy settings of applications on your phone, and thoroughly check which ones you gave permission to.