Refactor mass assigning from a hash
To add something to Flambino's answer, I'd suggest moving any non-trivial code to the model (code is then easier to test, re-usable and controllers are kept simple):

class User
  def self.new_from_auth(auth)
    attributes = [:first_name, :last_name, :username, :email]
    user_attributes = if auth 
      info = auth.extra.raw_info
      Hash[attributes.map { |attr| [attr, info.send(attr)] }]
    else
      {}
    end
    User.new(user_attributes)
  end
end 

class UsersController
  def new
    @user = User.new_from_auth(session[:auth])
  end
end