Originally Posted by kalniel
Probably a misread. According to Blizzard's press release, the expansion sold 1million copies in the first month, while the original game sold 4 million in the first year. But that's not like for like. The same press release says they're about the same:
Though they only shipped 2million to retailers, so perhaps they were expecting it to slow down and actually hit about 50% at most.
I don't think the expansion to D3 will rival the sales of the original game, largely because we're in a different game market now, but I could be wrong, I do expect them to be at least as good as sales of the LOD expansion though.