http://frictionalgames.blogspot.com/2012/09/amnesia-two-years-later.html

They have pretty nice sales.
Without counting the units bought there our total lands at 710 000 units. Adding all HIB and Potato Sack sales gets us to 1 360 00 units in total, which can be called the optimistic figure. This means that, optimistically speaking, Amnesia has sold almost 1.4 million units!
Amnesia is as of writing the 2nd most popular game at ModDB and sports 176 finished mods.
something like what I said in last years post, which was:
Windows: 70%
Linux: 15%
Mac: 15%
Perhaps a little higher (a percent or so) on mac because the mac app store has been doing pretty well.

Hard to give any exact figures though for the same reasons that counting the total units sold is difficult.