From the Instapaper weblog:

Today, we’re excited to announce that Instapaper is joining Pinterest. […]

For you, the Instapaper end user and customer, nothing changes. The Instapaper team will be moving from betaworks in New York City to Pinterest’s headquarters in San Francisco, and we’ll continue to make Instapaper a great place to save and read articles.

Instapaper CEO Brian Donohue insists that there are no plans to shutdown or materially change the service in the short- or long-term. But you can never be too sure. If you rely on Instapaper, it would be wise to find an alternative that you can switch to if things ever start to go south.