Facebook (News - Alert) unveiled today that it has reached an agreement to acquire Instagram, a popular photo-sharing app for mobile devices. The move will allow users to post on other social networks, follow users not on Facebook, and to opt out of sharing on Facebook.

The total cost 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 CEO of Facebook, posted about the transaction on his Facebook Timeline (News - Alert), “For years, we've focused on building the best experience for sharing photos with your friends and family. 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.”

Zuckerberg and his colleagues at Facebook believe that Facebook and Instagram are two different experiences that complement each other. In order to do this well, Zuckerberg will be mindful about building on Instagram's strengths and features rather than just trying to integrate everything into Facebook.

“That's why we're committed to building and growing Instagram independently,” he said “Millions of people around the world love the Instagram app and the brand associated with it, and our goal is to help spread this app and brand to even more people.”

To Zuckerberg, the fact that Instagram is connected to other services beyond Facebook is important. He wants to keep features including 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.

Kevin System, CEO of Instagram, wrote a blog post on Instagram that said, “Every day that passes, we see more experiences being shared through Instagram in ways that we never thought possible. It’s because of our dedicated and talented team that we’ve gotten this far, and with the support and cross-pollination of ideas and talent at a place like Facebook, we hope to create an even more exciting future for Instagram and Facebook alike."

Unlike many of other Facebook's acquisitions, Instagram won't be going away.  Instead it will be built into Facebook's network and mobile photo experience. 

In related news, Android (News - Alert) got the Instagram For Android app just last week. After Instagram first swept away iPhone (News - Alert) owners with its artistic and social photo-perfecting application, the startup is has finally released a version for Android users.