The thing with Facebook is that a while ago, it shut down its API (you can read more about it here), thus presenting users and developers with a restrictive new 3rd party app policy.
As a part of that, it sets the condition that in order for a 3rd party app, like drupe, to have access to your Facebook friend list, your friends must also give permission to be viewed by drupe, by installing drupe themselves.
Expecting all your Facebook friends to give permission to drupe is impractical for obvious reasons, hence the Facebook app is not featured on drupe.
We know this is a real drag, but we have virtually no power of Facebook's policy.