user = User.objects.get(profile__facebook=fb_user.get('id'))
# The actual code for getting the user branches off in a try catch, I believe...
# so this just has to be reflected.

username = fb_user.get('username', '') or fb_user.get('id')

if user.username != username:
  try:
    user.username = username
  except UniqueBlabla:
    old_user = User.objects.get(username=username)
    old_user.username = old_user.profile.facebook_id
    old_user.save()
    user.username = username
    user.save()