Today, Instapaper creator Marco Arment announced on his blog that he has sold a majority stake of the app to development house and accelerator Betaworks. Arment says that the deal is something that he’s been pondering for some time, as the app has grown.

This is basically an acquisition, and they will be running the service and taking over development of the apps. Arment says that the deal has been structured with “Instapaper’s health and longevity as the top priority, with incentives to keep it going well into the future.”

We asked Arment about whether the competition from other services like Readability and cross-platform Pocket had anything to do with the sale. “The deal had a lot more to do with Instapaper itself than the competition, honestly,” Arment told us. “I’ve been having a lot of trouble just keeping the product fresh, functional, and up-to-date for my existing customers.”

Arment says that he will continue advising the project indefinitely, but that Betaworks will take over its operations and development.

We asked Arment how it felt to be letting his baby go, the first product after his exit from Tumblr and his almost universal calling card for years.

“That’s a good question,” Arment said. “I really haven’t emotionally processed it fully yet, I’m sure. But for about 6 months, I’ve felt guilty with this burden of Instapaper’s continued development and maintenance that I’ve been handling pretty poorly. Now, I’m confident that I’m doing the best thing for Instapaper and its customers, and I feel great about that.”

Instapaper, if you’re unfamiliar, is a service that allows you to grab the contents of a page to read them later in a pleasantly reformatted manner. The service has apps for iOS, Android and a web interface. Arment developed the app by himself and launched it in 2008 as a web app and later an iOS app. But he says that it really needs a full-time staff to continue growing, and that’s not something that he thinks he’d be very good at.

Arment specifically says that he looked for a company that wouldn’t just shut it down in six months. “I wouldn’t put Instapaper in just anyone’s hands,” he says, “and I know that they’ll do right by it.”

You can read Arment’s full announcement here.

Betaworks acquired the flagging news hub Digg last year and has since relaunched it, growing it into a somewhat useful and relevant product again. Arment recently launched an iOS publication called The Magazine, which pays writers based on the subscription fees from each issue.

More to follow