Until now, the only way you could earn Orange Slices was from purchasing libraries. We've always meant for there to be more ways to get Orange Slices, though. Just now, we've launched our new Slices for Surveys program, which--as the name implies--is a way for you to earn Orange Slices in return for answering survey questions. These questions will allow us to better tailor our libraries to suit your preferences.
There are a few surveys up now, and we'll be adding more as time passes.
You can access the Slices for Surveys page in your account here.