Hmmm, I'm not sure where I found 8 million. So let's assume my bum!
It was up to 4.7 million in a couple months: http://www.wired.com/gamelife/2008/11/fallout-3-launc/
If you believe unreliable VGCharts, the combined 360 and PS3 sales top out at around 6 million.
http://gamrreview.vgchartz.com/browse.php?name=Fallout+3
Mass Effect 2 may be a follow up, but I wouldn't be surprised if Fallout 3 sold more than ME and ME2 combined. ME2 is a follow up to a game that sold decidedly less. Even if you just focus on the 360, New Vegas is a follow up to a better seller still.
And New Vegas did a good job of not deviating too much from the FO3 formula. So I wouldn't at all be surprised if New Vegas continued to see success from the "non hardcore RPG" market <gasp>.