In the Modules for Social Networks group on Groups.drupal.org I saw what might be the best simple answer for when to use Drupal 5 and when to use Drupal 6. Drupal.org promoted Drupal 6 heavily. I tend to steer clients away from Drupal 6 for immediate use as all the modules aren't ready  yet and it's not as thoroughly tested in the field as Drupal 5. But the user IceCreamYou said it all so well:

"The general trend is that people who run production sites without doing a lot of development first use last-gen systems because they're stable and well-tested. People who don't need a lot of complex functionality immediately (like with personal blogs) or people who want to test and develop on the new "toys" use the newest versions of everything. So social networking sites running on Drupal are mostly still on 5.x."

It's like poetry. It should be on the homepage of Drupal.org. It should be printed on tee shirts, put in inflight magazines and chiseled into granite near fountains at shopping malls.