over the company's collection and use of user data.

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What does the facial recognition do?

June 2011 was the date on which Facebook rolled out its "tag suggestions" feature.

The feature suggests who might be present in uploaded photos, based on an existing database of faces.

In Judge Donato's ruling, he laid out the four-step process behind the technology:

• Initially, the software tries to detect any faces in an uploaded photo
• It standardizes and aligns them for size and direction
• Then, for each face, Facebook computes a face signature - a mathematical representation of the face in that photo
• Face signatures are then run through a stored database of user face templates to look for similar matches

On its help pages, Facebook says the face templates are made from information about the similarities in every photo the user has been tagged in.