BoardTracker, the message board search engine, is now displaying time series of the number of boards and forums that it tracks. I've been monitoring these numbers for a while and it is interesting to compare different views. Here is BoardTrackers graphs of forums and threads over time:
The data I've been accumulating is over a wider time window:
My graph for forums suggests that BoardTracker updates its index of forums on a weekly schedule.
BTW, an additional stat I'd like to see is the number of actual boards that are crawled (not just the number of forums).