Possibly several people, I know I primarily have steam on my netbook for the chat thing, if they'd release an add in so you could use steam chat on trillian I'd be set

You really do have to wonder how many Steam users are actually buying and playing "real" games on it on a regular basis. I know a lot of people in real life who have a Steam account because of some sale on some game a blog linked to and they bought it even though they play on consoles 99.9% of the time. I am sure there are others only playing popcap stuff, only chatting or only using it for Steamworks games.
Anyway... it's interesting, and Valve will never tell.
I did notice the other day that when the Betrayal of Jimmy DLC for Mafia 2 came out it was the top seller all day and then when I played it late that night the leaderboards had 8,000 or so people on them total. So, 8,000 was an easy top seller that day, take that for what you will.

You cannot tell exactly but looking at their stats page, one sees that the number of concurrent steam users oscillates between 1.4 and 2.7 millions but when adding up the number for top games by current player count (top 100) you obtain less than 400k (current) up to 700k (peak today). I made this just now and used blocks of 10 and took always the maximum in each group to get an upper bound while speeding up calculation. What does it mean: approx. 70% of concurrent steam users are not playing a game. Thats my interpretation.
http://store.steampowered.com/stats/
For the estimated 3.500 Win 2000 users, the number of users wanting to play a game at some point and not having another computer with another OS available... thats really something nobody knows.
Mafia II users (peak today) were at most 20k simultaneously, now its down to 13k, and probably declining. If you would now know how long people play the game on average you could maybe infer estimates of sales numbers.
At least one can say that CS and CS:Source are still the most popular games on Steam.

Interesting but that'd have to be skewed dramatically by the fact that steam is set to start with windows so that 70% might realistically be people who just have their computer turned on and haven't told steam not to load on startup

Not quite. Just means only 8000 people were playing when you checked that night.
I know that with Dragon Age DLC I mostly bought it on release, and would play it a day or two later. On the first day, I would buy it and let it download and install in game, but maybe just read a book that evening. No reason Mafia 2 would be any different.

Uhh, no. Whenever you play you get on the leaderboards, you don't have to play right then or so anything special, it's automatic. 8,000 played it that day, period. Last I looked it's up to 16,000, but that was a couple days ago.

I dread the day I can't play my GOG games on XP . . . Oh, wait, they're DRM free . . . sorry . . . ;)

But you still could have had 40,000 purchases, it is just a question of WHEN they get around to playing it.
But if you would rather imply that Steam is dying, feel free :p

I am not implying anything of the kind, I simply reported the numbers. I guess a ton of people could have bought it and not launched it that day, but I doubt it would more than double the numbers.
Implying Steam is dying would be insane, since it is taking over PC gaming slowly but surely. I don't know how you got that from my comments. Just because I dislike Steam's DRM system doesn't mean I am some insane hater of the program like people seem to want to believe.

Its always difficult to judge anything when the numbers are not known or only partially known. I personally think that there is no reliable number out about sales and profit figures apart maybe from the annual business reports of exchange-listed companies. So Activision and EA maybe... somebody could look them up.
Nobody knows how much market share Steam or Impulse really has. There are only guesses. Even that Steam is taking over is just a guess. How good the competitors are is also a guess.
The number of people concurrently playing Mafia 2 or Empire Total War seems to my eye a little low (around 5.000 - 15.000). If I assume that each of this game is sold several million times... it seems like people are playing their games like only once per week on average or even less often?
On the other hand, Steam has a unique position with its plattform and DRM thing and obviously is growing in user base, and a lot of people are just DRM unaware and think Steam is the greatest thing since Rock'n'Roll which it isn't.
So, I am really interested in what will come out of this development and when (or if ever) we will see a reliable number published.

Well I say Steam is taking over because every major retail release this year seems to have Steamworks built in to all copies. It's a pretty dramatic rise from last year and before that, and very few people seem upset about it.
I don't know if that actually points to high Steam sales numbers or just the fact that it's the DRM people bitch about the least and offers DLC avenues.

But still its not sure. If being the biggest fish is everything Microsoft should have created the IPhone or MySpace should have dominated over Facebook.
However, the IPhone is a good example of what Steam probably wants to become, a closed plattform with some gimmicks and easy to use. And once you have a monopoly like Microsoft on OS or Facebook on social web, its hard to impossible to break it.
So, lets settle on: Steam has really big chances. :(

But now at least games with steamworks are being sold on other DDs with steamworks removed.

Yes, I love that. I hope that becomes standard.

It will. I'll support the digital download services that provide it.