No word yet on how much they spent but Facebook has acquired Face.com today. Why? Hit the jump for details on the deal.

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On Monday, Facebook announced that it would acquire Israeli startup Face.com for an undisclosed amount.

Face.com develops a mobile face recognition platform that can be used in various online and smartphone apps. The implications for how this could affect use on the world’s largest social network are huge—it may soon become even easier for mobile phone users to tag people, blurring the line between the physical and online worlds even further.

“The idea of using facial recognition to help with photo sharing was spurred during geeky garage parties, where it seemed that while many photos are taken, only a few get discovered or shared because only a fraction got tagged,” the company writes on its Frequently Asked Questions section of its site.

“By the time we released Photo Tagger, over a billion photos were being uploaded on Facebook alone each month, and we wanted to help users share photos more effectively with their friends. Following our release of Photo Tagger we launched Face Alerts, a feature which alerts you whenever new photos of yourself are uploaded by your friends, even before it gets tagged. It was a huge hit by users who were interested of keeping track of which photos of themselves got uploaded.”

Last month, Face.com released an iPhone app called Klik, which can scan photos to develop a facial profile of people in a certain photo set, or social network. Since 2010, the company has made its technology available to around 45,000 app developers for use within their own programs. Reuters put the valuation at around $60 million, as did TechCrunch.

“Our mission is and has always been to find new and exciting ways to make face recognition a fun, engaging part of people’s lives, and incorporate remarkable technology into everyday consumer products,” wrote Gil Hirsch, the company’s CEO, in a blog post on Monday.

Not surprisingly, neither company has been exactly forthright with how all of this facial data is used. Significant privacy questions remain.

“I am concerned about Facebook’s acquisition because Face.com has said it’s database includes about 40 billion face prints, and it is unclear at this point what Facebook plans to do with this data,” wrote Jennifer Lynch, a staff attorney with the Electronic Frontier Foundation, in an e-mail sent to Ars on Monday.

“We already know that the federal government regularly asks for copies of all photographs in which a user is tagged when it issues a warrant to Facebook. We also know that the FBI is currently building out the face recognition capabilities of its Next Generation Identification biometrics database and so has an interest in acquiring as many face images as possible. Face.com’s data could become a honeypot for law enforcement and marketers if Facebook doesn’t take steps to protect it properly.”

But the company seems to be dismissive of such concerns, despite the fact that the company’s privacy policy does not address facial data at all. CEO Gil Hirsch told Reuters in May that the company’s system “has been engineered from the get-go to preserve privacy and also deliver a social fun value and nothing creepy.”

Ars Technica