The data is drawn from a 3 month survey of ad impressions on Chitaka 's mobile ad network in North America. This of course means that it only tracks one of the activities that tablet owners use their tablets for and as a result this doesn't give a clear picture of market share, but the data does present strong evidence that the Kindle Fire is outselling tablets made by Samsung and Google - in this one market, at least.
But browsing the web is just activity, and if past Flurry reports are accurate then it is also the least common activity for Android tablet owners. They're much more likely to be in the Facebook app or to play games than to browse the web.
And as you can see from the graph, this report only covers tablets and not Android smartphones, where Samsung dominates.