First of all, let's see the facts.
- Sharing user's friendship data with all applications was wrong decision. There were a lot of bad actors. - So Facebook decided to stop this practice. But in the process some good actors are also burned, so they implemented some system to share some limited data considering:
- You have some critical mass of users - sign a special agreement to promise you will not use this data in a bad way - if having not access to this friendship data breaking your app's model - and probably your app is not competing with facebook's business model
so twitter came, agreed on this terms, got whitelisted. Then probably stepped over fb's toe with vine, and facebook revoked their access.
The thing is facebook didn't remove because it was competitive, they didn't allow competitive in the first place. This is not different from Apple not allowing another browser engines, or another app stores in their ecosystem.