Blizzard has revealed that Reaper of Souls has shifted 2.7 million copies in the first week of release through both online and retailers. Mike Morhaime had this to say about the numbers.
With all the new features added in RoS, sales should continues to rise as players help spread the word about all the improvements that have been added in the expansion.
A fun fact. Players have so far defeated Death more than 1.5 million times since launch.