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At least some of that traffic comes from preinstalled OEM copies, which Google pays the OEMs for having Chrome preinstalled as the default browser.
Also, it comes bundled with things like Java, Acrobat Reader,CCleaner, Divx Player, RealPlayer, Avast and Flash etc., so if people just click next, which most tend to do, they end up with Chrome on their machine. I think it used to come with Skype before Microsoft bought it.
http://mobilesociety.typepad.com/mobile_life/2011/09/outrage...
http://www.ghacks.net/2012/02/26/avast-7-will-install-google...
http://www.osnews.com/story/25184/Adobe_Tricks_Users_into_Do...
http://www.salsitasoft.com/2011/09/23/wonder-how-chrome-is-g...
Not to mention heavily pushing Chrome on Google properties like the search page, Youtube, Gmail etc. etc.
Google chat once hung in my Gmail tab in Opera, and when I reloaded it I got the message which said something like "Slow browser? Switch to Chrome.."
So Google's effectively buying marketshare with their search profits to cut out the middlemen like Firefox and Opera. Browsers like Firefox, Opera do not have the resources to push their browser this way and get people to try them.