That’s what I would describe as “the hopes and dreams of a generation” (of Software Freedom lovers), and is the result of fantastic product definition, branding, genuine user excitement, years of incredible — and largely unsung — work of thousands of Debian developers (not to mention all the upstreams)… and a pretty substantial X factor.
Update: Whenever I talk about this chart in presentations, I always follow up with another chart which puts that incredible rising line in context: Ubuntu vs. Firefox. Interestingly, that chart is looking far less impressive today than it was last year. Which is great news… The rise and rise of Ubuntu is catching up to the star power of Firefox! Wow!