Rihanna is most definitely overestimated. She was at 11.4M in October 2012.
Since then, she's had:
Diamonds - 1,000,000 (part of it is already included in the 11.4M number)
Stay - 600,000
Pour It Up - >200,000
Right Now - >200,000
What Now - >200,000
The Monster - 600,000
Can't Remember To Forget You - 200,000
FourFiveSeconds - 600,000
Bitch Better Have My Money - 200,000
American Oxygen, Towards the Sun, Jump, Lov7 Song, etc. were all big flops.
That adds up to 3.8M. I don't see how she could have more than 5M sales since October 2012, let alone 8.6M
Rihanna
madonna 
