When you sign into Facebook or Flickr whilst using Flock you will have access to a list of your friends via the People Bar.
Flock does this by calling the Facebook / Flicr API’s
Here is an example request as seen in a wireshark dump:
GET /restserver.php?query=SELECT uid,name,pic_square,status,profile_update_time
FROM user WHERE uid IN (SELECT uid2 FROM friend WHERE uid1 = 123456789)&
(flock facebook code: http://svn-mirror.flock.com/trac/flock/browser/trunk/mozilla/flock/base/services/ )
Flickr is accessed a similar way, via their rest interface – eg: