Update: Inc. profiled Kevin Systrom and Mike Krieger's start-up, Instagram, as part of our 30 Under 30 in May of 2011. Fast-forward 11 months, and their company has grown from 4 employees and 4 million users to a dozen employees and more than 27 million users. After word of accepting a new $50 million round of funding last week, Instagram today announced it is being acquired by Facebook for a staggering sum of $1 billion. Here's the story of this once-little app's humble beginnings.

Just after midnight on October 6, 2010, Kevin Systrom signed into his Apple App Store control panel. "Here we go," he thought. With a click, Instagram, the photo-sharing app he'd created with Mike Krieger, was open to the world. Beta users had been signing up for access for weeks, and now, with them all on board and posting photos, buzz was building. Fast.

"We crossed 10,000 users within hours, and I was like, 'this is the best day of my life.' This is amazing, right?" he says. "At the end of the day, it kept growing so much I thought, 'are we counting wrong?'"

They weren't counting wrong. Systrom estimates that 30 percent of the duo's collective energy was spent keeping the server up in the first weeks of Instagram's life. Thank goodness they had friends in high places. Krieger says he placed a lot of calls to "life-lines, Who-Wants-to-be-a-Millionaire-style" in those first days, including to (fellow 30 Under 30 honoree) Adam D'Angelo of Quora.

If you've marveled at the aesthetic quality of a friend's photo album lately, you can likely credit Instagram. The photo-sharing smart-phone application has been downloaded by four million people who use it to add stylistic filters, frames, and effects to photos, which can—by tapping one of 16 options—turn a straightforward snapshot of a housecat into what looks like a weathered Polaroid time-capsuled from 1977.

Systrom, a Stanford graduate who worked on Google's Gmail and corporate development, spent his weekends building an app that allowed location-aware photo and note-sharing, dubbing it Burbn. That's how Systrom met Mike Krieger, Instagram's co-founder: Krieger was an enthusiastic early Burbn user. Despite that the pair didn't know each other, they had both participated in Stanford University's Mayfield Fellows program, which educates students in successful and failed start-ups, and gives them internships with an established company and a start-up of their choice.

Burbn was pared down to photos only and dubbed Instagram. Today, it has more than $7 million in funding from Benchmark Capital, and the small company is coping with explosive growth in the app's user-base—up to more than four million—and scaling its mobile-social-networking components.

"Our goal is to not just be a photo-sharing app, but to be the way you share your life when you're on the go," Systrom says.

The duo, along with a customer service expert and another programmer—all four flannel-wearing 20-somethings—run a lean operation out of Twitter’s old office in San Francisco. "We're hoping some of the magic dust rubs off," he says.

Yes, that’s four employees for a company with four million users. Systrom says there’s no plan to monetize the app currently, as his team is "focused solely on the product and establishing ourselves as the leader in the mobile space." He admits he’s considering lots of different revenue models, including adding premium services such as extra filters or "Pro" accounts, or incorporating a full-scale advertising platform.