I'm getting tired of repeating this every time. WordPress.com does NOT power the 20% of the internet. The open source WordPress project does. WordPress.com represent a small % of all the WordPress installations.
My point is that these two sentences are totally true, but put together like that are misleading. And that's a bit discouraging for the non-automatticians contributing to core.
http://trends.builtwith.com/cms/WordPress
Quantcast Top 10k 1,066 of 10,000 - 10.7%
Quantcast Top 100k 11,664 of 100,000 - 11.7%
Quantcast Top Million 139,502 of 876,201 - 15.9%
BuiltWith Top Sites 408,954 of 1,974,430 - 20.7%
Most of Internet 12,177,314 of 192,744,298 - 6.3%