One measure of the momentum of the fine Drupal project is the number of people who are creating contributed modules on drupal.org.
The Drupal contributed projects are stored in a system called CVS and data about that is stored in some database tables that keep track of each change by each person. At the request of some fine folks who are working on important things, I got interested in the idea of the trend related to people committing code to the drupal.org CVS server. Here is the data graphed by the number of committers per month. It is not the number of commits, which would show how active those people are, but the number of people which shows how big of a group of people is doing this work.
Also, this is only about the contributed module and theme area and not about Drupal core. Drupal core commits are done by a very small group of people after that small group reviews the code contributed by hundreds of contributers. So, this really shows activity of the non-core projects.
I've labeled 4 points on the graph.
1. 2006 through Drupal 5.0 slump
Point 1 shows a peak at June of 2006 followed by a slow down until the trough at August of 2006 and then some small increases until December of 2006. Then there is a huge increase in people in January and February of 2007 which is also when Drupal 5.0 was released.