FACEBOOK is paying between $55m and $60m to buy Face.com, according to people familiar with the matter, acquiring the company that provides the facial-recognition technology used by the world's largest social network to help users identify and tag photos. The deal bolsters one of Facebook's most popular features - the sharing of photos - but the use of the start-up's technology has spurred concerns about user privacy.

Neither Facebook nor Face.com disclosed the terms of the deal, which is expected to close in the coming weeks. Face.com CEO Gil Hirsch said on his blog on Monday that "our mission is and always has been to find new and exciting ways to make face recognition fun. By working with Facebook directly, and joining their team, we'll have more opportunities to build amazing products that will be employed by consumers - that's all we've ever wanted to do."

Facebook, which will acquire the technology and employees of the 11-person Israeli company, said in a statement that the deal allowed the company to bring a "long-time technology vendor in-house".

Face.com, which has raised nearly $5m from investors, including Russian web search site Yandex, launched its first product in 2009. The company makes standalone applications that consumers can use to help them identify photos of themselves and of their friends on Facebook, as well as providing the technology that Facebook has integrated into its service.

Facebook uses the technology to scan a user's newly uploaded photos, compares faces in the snapshots with previous pictures, then tries to match faces and suggest name tags. When a match is found, Facebook alerts the person uploading the photos and invites them to "tag", or identify, the person in the photo.

Responding to inquiries from US and European privacy advocates, Facebook made it easier for users to opt out of its facial-recognition technology for photographs posted on the website last year, an effort to address concerns that it had violated consumers' privacy.

The deal is the latest in a string of acquisitions by Facebook in recent months, including the $1bn acquisition of mobile photo-sharing service Instagram. US antitrust regulators are undertaking an extended review of the Instagram deal, which Facebook expects to close by the end of the year.

Shares in Facebook, which continue to trade below the price at which they were offered during the initial public offering in May, closed in Monday's regular session up 4,7% at $31,41.