A question came yesterday up on the river-of-news list -- a list of developers who are creating tools and designs for rivers.
Dody Gunawinata, is doing an aggregator and wanted to know if he should support RSS 1.0 format. I said that River2 does, but I didn't know the breakdown of different feed formats.
So I wrote a script that recorded, when a feed updates, whether it's RSS or Atom, and if it's RSS, which version it is. Only feeds that updated in the 24 hour period would be counted, and each feed would only count once. The idea was to count feeds that were actually updating. Dormant feeds really don't matter for Dody's question.
I ran the script overnight on the server that maintains my rivers. So this is a very unscientific test.
The numbers: RSS 2.0 -- 338 feeds. Atom -- 36. RSS 1.0, 0.92 and 0.91 had 2 each.