Instapaper did not have a developer product that I'm aware of. It was a one-man product run by Marco Arment, and as I understand it, a successful one, but not successful enough to start employing a team of any real size.

It made most of its money through app purchases as the app was never free, and a small amount through subscriptions for a full text search feature that sold to a small group of the most active users.

I may be wrong on this, happy to be corrected. I'm going mostly off what Marco has said on his podcasts "Build & Analyze" and now "The Accidental Tech Podcast", which I have listened to on and off for a number of years.