Apple (AAPL) maintained its position as the No. 1 tablet manufacturer, but in battle for dominance among the Android tablet makers in the U.S. market — which is what ComScore is measuring here — Amazon is crushing Samsung.
In the worldwide market, where Amazon has a far smaller presence, the relative positions are reversed. According to IDC, Apple was still the leader with a 43.6% share of shipments in Q4 2012 , followed by Samsung (15.1%) and then Amazon (11.5%).
Take these estimates with a grain of salt since Samsung no longer reports sales or shipment numbers and Amazon never did.