People have recently found that they can request the data that Facebook holds on them, simply by downloading it as a zip file! Left completely shocked, people who have requested this have found out just how much data is stored – most would assume that Facebook would have data as to their habits on the messenger app, but it goes much further than that.
One person found that they hold an entire log of his call history with his partner’s mum, and another found that they have every single phone number in her contacts (Facebook user or not), data on every social event that she’s attended, a list of every text she has sent and a list of all of her friends and their birthdays!
This may not be as simple as case as users not setting their privacy settings up correctly; it seems to be fairly standard an occurrence. Your own data can be requested as a zip file here: https://www.facebook.com/help/contact/180237885820953
To read some further examples on what people have found, follow this link: http://www.iflscience.com/technology/people-are-downloading-their-facebook-data-and-are-horrified-by-what-theyre-finding/page-2/