I think it just problem this app, but it doesn't work:(
facebook app error
Let's hope so. Facebook's branding guidelines are a lot less stringent than Google's, but either way, I ended up using their logo asset and stylized the button with copy as recommended.
If I had to guess, I think their complaint was that FontAwesome wasn't using the right "Facebook Blue". That might've been enough, but I didn't see any harm in making a nicer looking button.