By all measurements, the new picture sharing service, 'Instagram, is exploding. A week after their launch, they had 100,000 users. A week later: 200,000. A week after that: 300,000. And then they were made Apple&'s App of the Week in the App Store. So yeah, basically they&'re likely far past a half a million users already. Not a bad first month at all.

But did you know that Instagram almost wasn&'t Instagram at all I detailed some of their transformation from the location-based HTML5 app Burbn to Instagram in my initial preview of the app. But co-founder Kevin Systrom gave more insight a few weeks ago on Quora.

Systrom revealed that he and co-founder Mike Krieger actually put in about a year&'s worth of work into Burbn, finally culminating in the building of a complete native iPhone app. But 'it just wasn&'t very good in their estimation. So they basically threw the entire thing out and started over. Again, after a year of work. The result, just 8 weeks later, was Instagram.

A huge risk. Ballsy.

&''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,&'' Systrom wrote.

That element to their story is fascinating to me. I was actually one of the initial users of Burbn after I got a tip about it and was able to sign up before Systrom realized who I was (he quickly emailed me later and asked me not to write about it until he was ready to show something off). I found Burbn interesting because location was getting red-hot at the time, and this was a completely-HTML5 approach that was pretty slick.

But I wasn&'t sure how such a service was ever going to take off. The reality right now is that we live in a world of native apps in the mobile space. And Burbn would have been coming very late to the game with an HTML5 gamble and with rivals like Foursquare, Gowalla and others already battling for users.

But it turns out Systrom and Krieger were thinking the same thing. &''Once he [Krieger] joined, we took a step back and looked at the product as it stood. By this time, we had built Burbn into a (private) really neat HTML5 mobile web app that let you: Check in to locations, Make plans (future check-ins), Earn points for hanging out with friends, post pictures, and much more,&'' Systrom noted on Quora. That all sounds great, but again, a crowded-space that was quickly getting more crowded.

&''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,&'' Systrom continued.

And so the pivot happened.

It&'s important to note that Systrom and Krieger had already secured $500,000 in funding prior to the pivot, so they had some cash to gamble with. But not all investors would be happy with such a turn. But as Systrom described it, 'Baseline Ventures and Andreessen Horowitz basically told them that they had some good ideas and to figure out the company they wanted to build from those ideas. They decided that Instagram, and not Burbn, would be that company.

Startups pivot all the time. Some are forced to, some are not. 'Want another good example Twitter.

The podcasting startup Odeo (where, incidentally, Systrom was an intern back in the day) was more or less going nowhere when Evan Williams told his employees to break into teams to brainstorm some crazy new ideas. One of those teams had Jack Dorsey in it, and Twitter was born.

Speaking of Twitter, Seesmic started as the &''video Twitter&'' before they pivoted to become a Twitter client. Now they&'re sort of pivoting again to cater more directly to the enterprise crowd. It&'s something that very well could work.

Meanwhile, the Y-Combinator-backed Fliggo earlier this year pivoted away from being another kind of video for Twitter and became Treehouse, an Instagram-competitor. Funny how it all comes around. Will they catch on like Instagram has Who knows, but if not, the entrepreneurs behind it will undoubtedly be back at it with another idea.

Instagram&'s story is a testament to the fact that pivots can and do work �a4���a4� even if it means tossing much of what you had poured your life into for a year or more. It&'s all about timing, and learning, and vision, and a hell of a lot of courage.