Want to know how Instapaper grew into such a wildly popular app for so many iPhone and iPad users? Read this article from Marco Arment, written two years ago, just after the release of Instapaper Pro in the App Store:

Right now, I face a fork in the road: do I continue iterating and improving Instapaper.app, or do I start making other applications and hope for multiple income streams? Instapaper.app is at a relatively stable point. I can stop here and be proud of where I’ve taken such a simple idea. And, theoretically, I’d keep making some money with Instapaper Pro while I work on something else.

But I’m not going to stop here. […]

I want Instapaper to be the essential app for every iPhone and iPod Touch user. I want it to be on every Apple geek’s short list when their friends and family ask them what apps to install. I want it to be one of your bottom four icons.

“The Future of the Instapaper iPhone App”