I just looked at an old TechCrunch article Did Twitter Just Pass Digg? [January 20th, 2009] The author, and many of the commenters, cannot believe that Twitter caught up with Digg. I wish I had seen this article earlier. I cannot believe that they didn't see how FAST Twitter was growing. It is immaterial whether Twitter surpassed Digg in January of 2009 or a month or two earlier or later.
To be fair to the writer, and the commenters, Twitter and Digg have two very different models and their usage is difficult to compare. How does one compare a tweet with a news aggregator such as Digg?