The iPad accounts for 88% of US tablet traffic

"Not all tablets generate the same click rate. Apple's iPad generates the majority of tablet traffic, with an 88% share, while Amazon's Fire managed to step in as a low-priced alternative, generating slightly more than 4% of tablet ad clicks in late December before dropping to 3.5% at the end of January.
The traffic from non-iPad tablets still comprises less than or around 1% of total traffic for most. In January, RKG estimates the traffic at 0.8%.
About 68.1 million tablet units shipped worldwide in 2011, up from about 19.6 million in 2010 worldwide, according to the IHS iSuppli Worldwide Tablet Market Tracker Q1 2012. The research firm estimates tablet manufacturers will ship an estimated 124 million units in 2012."

Source:  MediaPost, 8th February 2012