I have always been surprised by the relatively low rate of twitter referrers, 13k out of 6 million is like 0.2%, which a similar number to what I have observed, to get noticeable traffic you really need a few big twitter users to mention it and a constant stream of follow on / retweets.
its surprising since I follow a relatively small amount of people and check out most of what they recommend, I guess I am not indicative of the average twitter user