Google has an aggressive approach to marketing Chrome. Most obvious is the one on their own homepage, where certain browsers in certain scenarios (like IE9 at times, Midori, Sleipnir... the smaller browsers) have trouble keeping the add away once you X the add. Any "smaller" browser, once you end your session and then open a new one, will display the add again when you reach Google.
It used to be marketed on YouTube, and is included as a default checked option when installing software like Avast and (quite) a few more. Those who don't pay attention to what they're installing suddenly have Chrome as their default browser; some of them don't even notice, believe it or not - I've had many cases like that xD
It's no wonder why Chrome's usage soars so "quickly"...