Mocking up the interaction w/ FB like user246 suggests would probably be the fastest route as far as getting things set up and having full control over your environment.
But if you want real interaction with FB accounts, they do allow for test account creation as long as those accounts are linked to a main developer account responsible for them. Those test users are not visible to the public at all, and neither are interactions with them. Those test users can also be created pretty quickly as long as your using their Graph API to access FB data.
Info on how to do that is here: http://developers.facebook.com/docs/test_users/
I hope that helps!