[hmsimha]

I've seen different figures (http://trends.builtwith.com/cms/WordPress), but I'd say 1 in 5 doesn't sound too far off, though perhaps it's a little exaggerated.

[mseebach]

In a long tail world, it only really matters if you're in the tail or the "body". If you're in the body, there's no big difference if you're 1 in 5 or 1 in 50 - you're huge.