stevem brings up an important point - MusicCity's abandonment of the OpenNap system - that I did not address in the article.
According to CNET (http://news.cnet.com/news/0-1005-200-5714860.html),
MusicCity ceased use of OpenNap on April 22, near the end of our study period. However, the first illustration in the article shows the reported decline happened over a stretch of time prior to that singular event. This decline was therefore independent of MusicCity's actions.
In fact, although it appears they were unusable at the time, MusicCity's OpenNap servers continued to appear "alive" to our monitor after April 22, so the data do not show a sharp drop associated with their shutdown. If we had included this event, the total decline would have been even greater.
Earlier this month, MusicCity's former OpenNap servers dropped off our monitor, and they are no longer included in the count or list presented at the Clip2 site.