*Update* Facebook has amended this issue through a patch.
If you use Facebook, your phone number may not be as private as you think.
A way in which Facebook privacy can be abused has come to light that will shock many users, but that the social network itself seems to consider a deliberate feature.
If you enter someone's phone number into the search box on Facebook, the site can perform a reverse look-up and tell you who the phone number belongs to.
You can see in the screenshot how I entered the mobile phone number of someone I am not Facebook friends with, and instantly was offered their name, photograph and a link to their profile.
When I spoke to the Facebook user in question, she was shocked and surprised that I had been able to find her profile simply by entering her mobile phone number.
She confirmed that her privacy settings were correctly locked down to such an extent that her phone number should only be accessible to her.