Keith Bedford/Bloomberg News

Facebook is not waiting for its initial public offering to make its first big purchase.

In its largest acquisition to date, the social network has purchased Instagram, the popular photo-sharing application, for about $1 billion in cash and stock, the company said Monday.

It’s a notable move for Facebook, which has exclusively focused on bite-size acquisitions, worth less than $100 million.

With Instagram, Facebook will get a formidable mobile player – an area that is seen as a weakness for the sprawling social network. Founded two years ago, the service — which lets users share photos and apply stylized filters – has become one of the most downloaded applications on the iPhone, with some 30 million users. Instagram released a version of its application for Google’s Android operating system last week.

On Monday, both companies expressed their commitment to run Instagram as an independent service.

Pool photo by Yuriko Nakao

In a post on his profile page, Facebook’s chief Mark Zuckerberg said Instagram would continue to work with rival social networks. That will allow users to post on other services, follow users outside of Facebook, and to opt out of sharing on Facebook.

“For years, we’ve focused on building the best experience for sharing photos with your friends and family,” Mr. Zuckerberg wrote. “Now, we’ll be able to work even more closely with the Instagram team to also offer the best experiences for sharing beautiful mobile photos with people based on your interests.”

In a separate blog post on Instagram’s Web site, the company’s chief executive, Kevin Systrom, also reiterated plans to preserve the service’s functionality and said he looked forward to leveraging the new parent company’s resources and talent.

The announcement comes as Facebook prepares for its highly anticipated initial public offering, widely expected to take place next month.

Though Facebook is known for smaller acquisitions, Instagram’s surging momentum likely compelled the social network to swiftly put together a billion-dollar offer. Last week, Instagram, which has just a handful of employees, closed a financing round worth more than $50 million with several prominent investors, including Sequoia Capital, an early backer of Google, Thrive Capital, the firm run by Joshua Kushner, and Greylock Capital, an early investor of LinkedIn. AllThingsD first reported last week that Sequoia was in the process of leading a $50 million round in Instagram.

That latest funding round valued Instagram at about $500 million, according to one person with knowledge of the matter, who requested anonymity because discussions were private. Facebook’s purchase, one week later, means that investment has now doubled in value.

The deal is expected to close later this quarter, according to Facebook’s statement.

Here is the news release from Facebook:

“Facebook announced today that it has reached an agreement to acquire Instagram, a fun, popular photo-sharing app for mobile devices. “The total consideration for San Francisco-based Instagram is approximately $1 billion in a combination of cash and shares of Facebook. The transaction, which is subject to customary closing conditions, is expected to close later this quarter.”

Mark Zuckerberg, founder and chief executive of Facebook, posted about the transaction on his Facebook page: