Yet it is quite different: It costs money once, and you get updates for free - and there's no in-game market of any sort, whereas the OP wants no money upfront and tries to get people to buy in-game stuff.
1 million - January 12th, 2011
2 million - April 25th, 2011
3 million - August 7th, 2011
4 million - November 7th, 2011
5 million - February 18th, 2012
6 million - not sure.
7 million - August 15th, 2012
8 million - November 14th, 2012
Disclaimer: it's my site.
Second source: http://www.destructoid.com/minecraft-surpasses-one-million-s...