I've been monitoring the statistics published on BoardTracker for a few days now. My hope had been to see the number of forums and the number of threads showing a steady, monotonic increase. However, as the graphs below demonstrate, something quite different appears to be happening:
The second graph, showing the number of forums tracked over time, is probably the key here. The fact that it appears to flip between certain values suggests not that there is a gradual purging of, for example, dead boards, but that there is some issue with BoardTracker's infrastructure.
What is interesting also is that during the period at the centre of the second graph during which there appear to be not additions to or removals from the database of boards tracked, the number of forums shows a steady increase: the natural growth of the forums on a closed set of boards.