Earlier this month we marked four years since Instagram launched. Throughout the day, I glanced at what time it was, and thought back to what we were doing four years ago:

6am: Biking through a misty San Francisco morning to our shared co-working space at Dogpatch Labs.

7am: Stomach in knots, Kevin and I scarf down bagels from Crossroads Cafe.

8am: Press embargo lifts, our first users come streaming in.

9am: Kevin and I panic as our tiny server crumbles under the weight of our first-day traffic.

12pm: Relief as we get things sorted out and back up.

2am: Still awake. 25,000 people have signed up.

6am (again): Taking the N train back to the Haight and collapsing.

That first day was a preview of the rest of the year; alternating between giddiness that we might have created something that really resonated, with panic that we wouldn’t be able to keep up with demand, and that the wheels might come off the wagon.

Late night Instagram office in the Dogpatch neighborhood of San Francisco, 2010.

People often ask how much of Instagram’s adoption we anticipated. Working on a startup is a balancing act: being crazy enough to believe your idea can take off, but not crazy enough to miss the signs when it’s clearly not going to. We grappled with both in the year before launching Instagram.

At first, we were building an app called Burbn, a location-based social network written in HTML5. Burbn was well-liked and had a few passionate daily actives, but it wasn’t exactly setting the world on fire. Our attempts at explaining what we were building was often met with blank stares, and we peaked at around 1,000 users. For those early adopters, though, it was a new way of sharing what they were doing out in the world. Many of our favorite updates came from friends who posted to Burbn after putting their photos through some early filter apps, compensating for the lower image sensors on phones like the iPhone 3G.

Kevin and I took a walk down the Embarcadero after an investor meeting, mid-July 2010, and came to a decision. Locking ourselves in the single conference room at Dogpatch Labs, we said out loud what had been bubbling under the surface for weeks: we needed to scope down the product we were building, or risk failure in trying to be too many things at once. It was time to try something different — why don’t we take the photo updates from Burbn and make them into their own product?

Our fundamental idea was that people would want to connect and share experiences out in the real world, through snapshots of their lives. In retrospect, Instagram may seem “obvious” — communication through photos is universal. But products are defined by a series of decisions and assumptions, and our combination of being photos-first and public-by-default would prove to be a combination that solved an unmet need.

Early Instagram sketch by Mike Krieger, 2010

Kevin and I spent much of the next week at Crossroads Café in San Francisco, sketching pages and pages of user interface ideas. Nuggets of UI ideas from Burbn made their way in; photos were large and centered, rather than just thumbnails. Comments were present in-feed rather than hidden behind a link, even though that made our life more difficult in terms of scroll performance. Within two weeks, we had our first version, including Kevin’s first pass at filters. We wanted to learn quickly whether this was something worth exploring further, so we took 100 of our Burbn testers and sent them this email:

“For the last 6 weeks, we’ve been working on a native iPhone app for communicating & sharing in the real world through photos. We’ve taken parts of our HTML5 app experience that got people the most excited and focused on them. This means there are quite a few features that we wouldn’t have been able to do in HTML5, but also means there are elements of the HTML5 app that didn’t make as much sense to carry over. Try to approach this experience as “inspired by webapp Burbn” ☺”

When you have a beta testing group of 100, all you can measure is relative success. Within a couple days of rolling Instagram (then simply called “Codename”) out to that test group, though, we knew we were onto something much bigger. Weekends had always been our peak days, and the first weekend that Instagram was in testers’ hands, the output of that small group already eclipsed what we were seeing on Burbn. Not all of our Burbn users loved it; some left and never came back. We had to drop Android support, at least for the time being, which meant one of our most-engaged Burbn users could not longer participate.

Those who fell in love, though, started documenting their daily lives with dedication, and after some polish and iteration with our beta testers, we launched to the world. Within a week, we were being surprised by how people were using Instagram. Halfway through day one, Kevin turned to me and said, “I don’t know how big this is going to be, but I think there’s something here.” Roughly 100,000 people signed up in the first week.