Instead of shutting down the service with an export option to a data file, how about identifying two or three competitors you'd recommend and working with them to export the content directly into another service. This way you help out both your customers and another startup.

For example, if Grove.io content can be made to work with Flowdock, contact the Flowdock team and work with them to put a big "Migrate to Flowdock" button on the shutdown announcement page.