Facebook LOVES to encourage you at add as much information as possible—education, hometown, date of birth, contacts from your address book and even your mobile phone number.
The point of this is, on a basic level, to make the app as user-friendly as possible, meaning you can easily contact your friends, find people you know and generally you know, be social on social media. Which there's no denying is pretty handy.
But it turns out that sharing your mobile number on the site is actually not the best idea ever. If you do, it means anyone can potentially find your name, picture and location just typing the number into the search bar, if that info is public. HMMM.
In theory, this means hackers could have access to all sorts of data, just with a simple phone number.
Software engineer Reza Moaiandin has explained how easy it was to use a simple algorithm to guess mobile numbers and run these through a program to find out which were linked to Facebook accounts. Then, it was pretty easy to discover all sorts of info.
"This could be a huge phishing problem if no limit is created, and the loophole is discovered by the wrong person," he wrote in a blog post. "Unfortunately, for the 1.44 billion people currently using Facebook, this means that sophisticated hackers and black market sellers can access names and mobile phone numbers in as little as an hour through reverse engineering."
So, how can you protect yourself?
Check your privacy settings ASAP. Go to Settings -> Privacy -> Who can look me up? and you can see 'Who can look you up using the phone number you provided?' and 'Who can look you up using the email address you provided?' Make sure it is set to either Friends or Friends of Friends.
We've got a whole separate page on Facebook tricks and tips you really need to know, from how to secretly block people, to finding secret messages in your inbox that you didn't know you had and seeing who has unfriended you. As well as all the privacy basics.
For more advice on the number linked to your account and how to remove it, there's more on Facebook's help pages about 'How do I add a new phone number to my account or switch numbers?'.
What has Facebook said about it all?
"The privacy of people who use Facebook is extremely important to us. We have industry-leading proprietary network monitoring tools constantly running in order to ensure data security and have strict rules that govern how developers are able to use our APIs to build their products. Developers are only able to access information that people have chosen to make public.
"Everyone who uses Facebook has control of the information they share, this includes the information people include within their profile, and who can see this information. Our Privacy Basics tool has a series of helpful guides that explain how people can quickly and easily decide what information they share and who they share it with."
This article originally appeared on Cosmopolitan.co.uk. Minor edits have been made by the Como.ph editors.