Arment said he had sold a "majority" stake in the service, and will remain an adviser to the project. He said he expected the new company to add staff and further develop Instapaper.
"With Betaworks' drive and resources now behind it, I'm confident that Instapaper has a very bright future," he wrote. "I'm looking forward to seeing what they can do."
Betaworks, a capital investment firm that concentrates on "social web" products--including a recent revival of the venerable Digg news-sharing service--offered little public comment, but pointed to Arment's blog post on its website. "We're thrilled," the company said in its announcement.
Arment, who co-founded Tumblr, launched the Instapaper service in 2008, letting users save long-form articles for later reading in their Web browsers and on e-readers like the Amazon Kindle. It was its arrival in the iOS App Store later that yer, though, that made it a hit: Instapaper doubled its user base during the first month its iOS app was available.
The service has 2 million registered user accounts, according to its official statistics.
In the last year, Arment had turned his attention to creating and editing The Magazine, a digital-only magazine--originally available only as an iOS app--that features just a few long-form pieces per issue. (Full disclosure: Several members of the Macworld and TechHive staffs have written for that publication.) The publication quickly, and prominently, became profitable.
Since Arment originally created Instapaper, competitors such as Pocket (originally Read It Later) and Readability have come along. Those other services have had the power of startup organizations behind them, while Instapaper had remained a solo project until now.
In his blog post, Arment said Instapaper had grown beyond what he could maintain on his own. "Instapaper needs a new home where it can be staffed and grown, but I didn't want to give it to a big company that would probably just shut it down in six months."
"A couple of months ago, at 1:30 AM, I suddenly realized who should take it over," he wrote. "I jumped out of bed, tiptoed downstairs...and sent an email (to Betaworks.) It didn't take much convincing, because we both knew it was a great fit."
The deal, he said, should ensure Instapaper's long-term health.
"We've structured the deal with Instapaper's health and longevity as the top priority, with incentives to keep it going well into the future," Arment wrote. "I will continue advising the project indefinitely, while Betaworks will take over its operations, expand its staff, and develop it further."
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