Why did ~20k Drupal 7 websites just drop off the map?
If you look back through the statistics they fluctuate quite often really. 

Looking at stats for May 26th 2013 (642,589 Drupal 7 sites) or June 9th 2013 (666,573 Drupal 7 sites) I'm not actually certain it can be taken to represent sites disappearing so much as people evaluating Drupal or indeed installing and uninstalling new development snapshot instances.

It's also well worth bearing in mind that the accuracy of the statistics is not 100% and sites which don't use the update manager module don't report in at all. (It is, in some circles, common practice to disable this component once a site goes live if it's being updated in a separate development environment).