Jhwisner sent news about two new articles were ArsTechnica has data on Steam sales. I will post what he sent to save me time. Allow me to say thanks Jhwisner.
More information.ArsTechnica managed to produce some apparently decent estimates of total ownership/sales totals on Steam. A few Devs have confirmed the accuracy with the author - some publicaly so. Imporant takeaways from this are pretty positive for PC gamers in general and PC RPG gamers in general.
Here's some interesting tidbits:
Skyrim - 5,942,000
Fallout: New Vegas - 2,630,232
The Witcher 2 - 1,725,513*
FTL - 1,651,734*
Dark Souls: Prepare to Die Edition - 1,368,606
*Only represents steam sales totals as DRM free versions without Steam integration are on offer on sites such as GoG.
Link - http://arstechnica.com/gaming/2014/04/introducing-steam-gauge-ars-reveals-steams-most-popular-games/
Second article contains an expanded top list - expanded to 100 games (page 2 of article)
Link - http://arstechnica.com/gaming/2014/04/steam-gauge-addressing-your-questions-and-concerns/