The service we were building (I work at app.net) was a social app discovery service. It integrated with Facebook to show you what apps your friends were using and downloading, and display stories about the apps you use.
Aren't you always extending Facebook's core features when building with their API? The point of leveraging Facebook platform is to build amazing products that Facebook doesn't have the time or resources to create (see zuck's statement on the importance of Facebook platform during the earnings call).
We used Facebook's APIs and Open Graph in our app discovery service.
It was built and released before Facebook's App Center was announced, and Facebook dev-relations employees had previously been supportive of our endeavor.
I'm pretty sure I've seen this somewhere else... at least a dozen times.