Instagram CEO Kevin Systrom insisted that "Instagram is not going away."
"We'll be working with Facebook to evolve Instagram and build the network," Systrom wrote. "We'll continue to add new features to the product and find new ways to create a better mobile photos experience."
The app will remain in the App Store and on Google Play and users will still be able to share on social networks other than Facebook, Systrom continued.
"Every day that passes, we see more experiences being shared through Instagram in ways that we never thought possible," he continued. "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."
As Fast Companynoted last year, Systrom turned down an offer to join Facebook back in 2004.
In his note, Zuckerberg said Facebook has focused for years on "building the best experience for sharing photos with your friends and family." The Instagram acquisition will "offer the best experiences for sharing beautiful mobile photos with people based on your interests," he said.
Zuckerberg reiterated that Facebook will be "building and growing Instagram independently."
"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," Zuckerberg wrote. "We will try to learn from Instagram's experience to build similar features into our other products."
The Instagram team, though, will have access to Facebook's engineering team and infrastructure.
Zuckerberg noted that the purchase is notable because it's the first time that Facebook has acquired a company with such a large user base - at least 30 million, Instagram said last week. "We don't plan on doing many more of these, if any at all," he said.
Instagram for Android was released last week and quickly nabbed 1 million downloads. Instagram for iPhone debuted in October 2010, and let users add different custom filters to photos in order to change the colors, mood, border, and tonality of their snapshots. It hit 1 million users after just three months.