Mark Zuckerberg announced via Facebook today that his company is buying Instagram, the preferred mobile app of everyone who photographs their food, for $1 billion in cash and stock. "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," says Zuckerberg. "We don't plan on doing many more of these, if any at all." Last week, Instagram expanded from the iPhone to the Android platform, adding more than a million new users in half a day. Then, just before the acquisition, the two-year-old San Francisco startup grabbed $50 million in new funding. Good week!

Both Zuckerberg and Instagram CEO Kevin Systrom insist the photo-sharing/filtering service will continue to grow independently. "We think the fact that Instagram is connected to other services beyond Facebook is an important part of the experience," Zuckerberg writes. "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."

A New York Times feature about Instagram last year, back when they had just five million users (compared to 30 million now), noted valuable account-holders like MTV, Starbucks, Kate Spade, and Snoop Dogg. Zuckerberg reportedly reached out about a potential acquisition at the time, but was rebuffed. Today, Systrom writes as a much richer man, "We’re psyched to be joining Facebook and are excited to build a better Instagram for everyone."