Not on the actual Kindle itself, but on mobi/KF8 sells I saw an article several months ago which based its estimate on publishing house sales figures. As we know Amazon doesn't talk about numbers, but several of the large publishers do. The article examined the publishers' figures and determined that in the USA mobi/KF8 makes up 67% of the sales for all eBooks. That means Amazon which is the sole seller of mobi/KF8 commands 2/3 of the market leaving 1/3 to all the other bookstores to share. I think you can interpolate from the eBook sales how much of the market Kindle commands.