Got the reply from on the Google Group (http://groups.google.com/group/android-developers/browse_thread/thread/95110dd7a1e1e0b4):
Access to Facebook friends via the
contacts provider is restricted to a
handful of system apps by the provider
itself. Other applications cannot
read that data.
Therefore now I fetch an cache all contact names/id mappings via FB Graph api (http://graph.facebook.com/me/friends) and use that for the id lookup.