Trying to extend Facebooker::User

I have been playing around with the Facebooker plugin (and am brand
bew to Rails to boot). I am trying to add methods to Facebooker::User,
by making my own Facebooker:User model, i.e.:

class Facebooker::User
  def lists
    123
  end
end

But when I access facebook_session.user.lists, I get a no-such-method
error. I have only played around casually with Ruby before, but was
always able to add methods to classes by "reopening" them with no
problem; why would this case be any different? Thanks for any advice
(or Google queries I didn't think of:))