The graph above is made based on the project usage statistics collected on drupal.org. As ever with statistics of this sort, they don't tell the whole story. This is because only sites running the update status module report data back to drupal.org. This module is part of Drupal 6 and the installer prompts the user to enable the module when Drupal is first installed. It is not required to enable this module. People upgrading from Drupal 5 aren't even prompted to enable it. Plus, many Drupal sites are hidden behind corporate firewalls. As a result, we don't really know how many Drupal 6 sites there are.
Either way, based on the growth data that we do have available, we can predict that we will near 240,000 Drupal 6 sites by January 2010. See the black trendline on the graph. The R-square represents the variability in the data set that is accounted for by the prediction model. Its value indicates how likely the predicted values are -- the closer to 1.0, the better. If Drupal 6 continues to grow like it did the past 9 months, our prediction should be pretty accurate. It would mean that the number of Drupal 6 sites will double over the next 9 months. Not bad.
I always believed that the best way to grow Drupal is to make the software better, and that is why we continue to work hard on Drupal 7. But until Drupal 7 is released, there are a lot of things that we can do to help people get started with Drupal 6 -- from offering .zip-files instead of .tar-files, to launching the drupal.org redesign, to sharing more Drupal 6 success stories, and more. There are a lot of barriers that could be removed and that would result in faster growth.
Once in a while, it is good to make lists. You're all invited to share your list of "things you think we should do" in the comments of this post. I recommend that you prioritize your list so the most important item is at the top. If you are actively working on any of the items on your list, let us know too. Bonus points for ideas that have high impact, require minimal effort and benefit the Drupal community at large. Penalty points for people that recommend Drupal 7 features based on self-interest. Ready, steady, go!