Facebook is spending $US1 billion ($A978 million) to buy the photo-sharing company Instagram in the social network's largest acquisition ever.

Instagram lets people apply filters to photos they snap with their mobile devices and share them with friends and strangers. Some of the filters make the photos look as if they've been taken in the 1970s or on Polaroid cameras.

"This is an important milestone for Facebook because it's the first time we've ever acquired a product and company with so many users," CEO Mark Zuckerberg wrote on his Facebook page. "We don't plan on doing many more of these, if any at all."

Facebook said it plans to keep Instagram running independently. That's a departure from its tendency to buy small start-ups and integrate the technology - or shut them down altogether just so it can hire talented engineers and developers.

"We think the fact that Instagram is connected to other services beyond Facebook is an important part of the experience," Zuckerberg wrote.

"We plan on keeping features like the ability to post to other social networks, the ability to not share your Instagrams on Facebook if you want, and the ability to have followers and follow people separately from your friends on Facebook."

Facebook is paying cash and stock for San Francisco-based Instagram and hiring its roughly 10 employees. The deal is expected to close by the end of June.

Menlo Park, California-based Facebook is expected to complete its initial public offering of stock next month. Getting Instagram is a big win for Facebook as it works to harness people's growing obsession with their mobile devices and sharing every moment of their life.