So, while our fellow Russian friends are taking GOG over, one of them did a website with some black voodoo magic algorythms which estimates the number of owners per game with a big margin of error, apparently the numbers seems to corellate reality:
https://twitter.com/TomGrochowiak/status/583949020294475776

http://steamspy.com/

Yes. Seems like an interesting site for sales estimates. Doesn't say much about GOG share (obviously... :) ).

GOG share around 10-15% of Steam sales in general for the few numbers which have surfaced ;)
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BTW, dedicated thread from Neogaf where Zeboyd (Chtulhu saves the world, Breath of death) confirms the numbers
http://www.neogaf.com/forum/showthread.php?t=1022509

Interestingly, on page 5, the web developer says in Russian in his tweet that some devs who have a publisher reports that their sales number are way lower which means that some publishing companies (like TopWare which already screwed up one dev) could have diminished their sales reports in order to give a lower cut to the devs....
POPCORN TIME if it happns to be true! :o)

PS: of course, self-published devs knows their sales number as they have direct access to steam back-end. Not everything is supposed to be accurate ;) Ars Technica said Unturned was the most played game on Steam (in their own "try to estimate steam sales" article) even if I didn't heard about this game beforehand....

EDIT: the "free" week-ends are taken into account.

This takes into account ownership only, and doesn't say where the game was purchased. For instance, a lot of the most-owned games had physical releases (Civilization 5, Skyrim, etc.). A lot of people could have acquired the keys from bundles, Green Man Gaming, GamersGate, direct from Steam, etc. so it's hard to measure how an individual store is doing, to say nothing of how much was paid for the game.

I guess that makes more sense.

That's why I wrote "unit sales" ;) A good example would be "Mark Of The Ninja" which was in a humble bundle. This is not at all a tool to measure revenues but it gives a good insight of the ranges of units "shipped".

BTW, even at 1$ per unit I wouldn't spit at that kind of money ;)

Bump with the website dev giving some details and disclaimers on neogaf:
http://www.neogaf.com/forum/showpost.php?p=158727832&postcount=329

Gamasutra's already on the news and devs like Lars Doucet are confirming on twitter. Even my previous estimations of "Seasons Of The Wolf" based on number of reviews got sadly confirmed :(

MotN was bundled with 3 games one of which was an AAA release though.

Up with Wadjet Eye comments on SteamSpy and its own sales:
http://www.wadjeteyegames.com/2015/04/05/lets-talk-steam-spy/

Steam Winter Sale: Recap by Valve

https://steamdb.info/forum/790/

Yep and here's the winter sales article by the SteamSpy guy:
https://medium.com/steam-spy/about-steam-winter-sale-76a75abe152a#.yze7z78is

(I'll read both articles later, surely must be interesting)

As a result of this format change, we were able to encourage customers to browse through their Discovery Queue, thereby surfacing a string of personalized recommendations including titles that aren’t otherwise highlighted on the home page. By dropping users a free Steam Trading Card for browsing through their personalized Discovery Queue (up to three times each day) many customers were exposed to 36 different product pages every day for each of the 13 days of the event.

I love how Valve think it was the format change that partly encouraged people to browse that silly discovery queue. They seem to downplay the fact that people will do anything to get those silly cards, just they can turn them around and make some easy wallet money. Had they followed the previously established model of daily discounts, flash sales, and still had cards awarded from doing that queue, people would have still been "encouraged" to do that queue regardless.

That said, the sale despite what Valve intended, still had a few flash sales. Only this time, they were harder to find and not always highlighted on the front page.

Why bother really, we all know Steam sales and Steam users dwarf anything else on PC.
Places like GOG are but asteroids going around a star.

I think it's comets that go around a star, not asteroids.