Our main activity is only for those who aren't using other plugins who also want the main activity.
In order for activities to play nice with each other, the better thing to do in your case is to create a class that extends CBUnityPlayerProxyActivity
and add:
@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);
Session.getActiveSession().onActivityResult(this, requestCode, resultCode, data);
}
This will pass data to the Facebook SDK correctly.