Facebook have turned on their new facial recognition feature for all accounts (it's been active in the United States since last year). This means that when you upload a new picture to Facebook, it is automatically scanned by Facebook's facial recognition software to try and match and faces in the picture with people already tagged in existing pictures on Facebook. If it thinks it finds a match, it will prompt the user uploading the picture to tag it accordingly. The feature is 'on' by default. To find out more, including how to turn off the feature, check out this article at The Register.
