To date people have questioned Facebook's mobile strategy. Zuckerberg started his social network in the days when PCs and browsers ruled the internet. Even 20-somethings can look a bit dated in these fast moving days. And $US1b is a small price to pay for new school cool, if you are worth $US100b.
Systrom, a Stanford University graduate like so many Silicon Valley multimillionaires, grew up in Boston but was an early witness to the dotcom boom. His mother, Diane Systrom, worked at Monster.com during the first internet era and is now an executive at Zipcar, the online car rental business.
The history of the billion-dollar deal goes back to his university days where he was studying for an engineering degree. Systrom, a big photography fan, started looking at ways to share photos online. His interest subsided as he looked for a job, ending up at Google, where he spent two years in product development and corporate development.
Systrom's next job was at Nextstop, a trip-recommendation site that Facebook bought for a rather measly $US2.5m. Systrom then started Burbn, named after his favourite liquor, a company that focused on the super-hot area of mobile but whose basket of services seemed to lack any clear identity. It had photos but also check-in capabilities, like FourSquare, and other apps.
Along came Mike Krieger, another Stanford graduate, and the two started talking about narrowing their focus.
On the Q&A site Quora, Systrom explained the genesis of Instagram: "We decided that if we were going to build a company, we wanted to focus on being really good at one thing.
"We saw mobile photos as an awesome opportunity to try out some new ideas. We spent one week prototyping a version that focused solely on photos. It was pretty awful. So we went back to ... Burbn. We actually got an entire version of Burbn done as an iPhone app, but it felt cluttered, and overrun with features.
"It was really difficult to decide to start from scratch, but we went out on a limb, and basically cut everything in the Burbn app except for its photo, comment, and like capabilities. What remained was Instagram. (We renamed because we felt it better captured what you were doing - an instant telegram of sorts. It also sounded camera-y)."
The rest is Silicon Valley history. Launched in October 2010, Instagram was an instant hit. Over 30m people have downloaded the app now. When the firm launched an Android version earlier this month, it attracted 1m downloads in 12 hours. People love sharing their photos online and making them look like their Dad took them in 1980 with a camera he borrowed from his dad.
And the app they want to do it with is Instagram.
Guardian News & Media and Fairfax Media
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