Given the success of The Magazine, hard not to see this happening soon. From the post on Marco's blog:
I’m happy to announce that I’ve sold a majority stake in Instapaper to Betaworks. 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. I will continue advising the project indefinitely, while Betaworks will take over its operations, expand its staff, and develop it further.
I was actually thinking about where Instapaper was headed a little while ago and recently had tried out Pocket, which is an Instapaper competitor/clone. I really like Instapaper but it has felt like the iOS app (Instapaper in the App Store) hasn't seen the design love that something like The Magazine has gotten.
I'll keep using Instapaper until there's something about it that either pushes me away or a feature in another app that draws me in. For now I don't feel like there's any reason to change and I trust Marco's judgement in who he has sold Instapaper to that they won't destroy what he's built any time soon.