As per Nazna's figures, plus my own observations of each, I suspect Steam is currently somewhat ahead of 360 (not but much) and definately ahead of PS3.

I also suspect it's DD sales are better than either Live or PSN looking at the breadth of both AAA big titles plus smaller, indie titles it has to offer.

So yes, it has more customers - love the Freudian slip though!

I'd assume that an active subscriber is a person that has a 'subscription', that is, owns a game on steam. Much like Daggy says. And I'm not all that impressed with Steams peak numbers either. Most people (my assumption) that installs steam doesn't untick the 'start steam when windows start' box in the settings, so the number of people active at the same time would be incomparable to the xbox's numbers, because people don't log in to Live unless they are playing or planning to play games (again, this is my assumption).

I'd agree with Steam being bigger than PSN, and I definitely think it brings in way more revenue than either PSN or Live, probably way more than both of those together (apart from the Live subsciption fee anyway). But I don't think, as the OP asked, that it has more customers than the PS3 or the Xbox 360.

Yes, the start on startup thing is something I have wondered as well for Steam's numbers. The number of people playing games via Steam is usually quite paltry. But that is why I think Steam is a much better source for DD but as a gaming portal, people don't use it as often.

2.5 million playing consequetively is "quite paltry" now?

I'm not really sure what you mean by consequetively. I can only assume that you either mean 'at the same time' or that you are making a joke. Not sure which, so I'm going to go with the first option and respond in kind.

That is the amount of users logged in at the same time. I'm usually logged on to Steam around 5 hours per day. I PLAY games on Steam around 1 hour per day. If everyone does the same, that would mean around 500k  people are playing games on Steam at the same time at the peaks. I bet the number would be much higher on Xbox Live.

Here are the top games being played on Steam as we speak and teh number of players.  First column is number of players now, 2nd column is peak.

I'm more interested in how many games are being sold on Steam than anything else.

It'd be nice if Valve would release data on that.

One thing I'd add, I guess it depends what you mean by customers. I'm not really considering online, as that will vary by days of the week, time of day, etc. but people using the service to play, buy games, etc. reasonably regularly.

Steam sells a fair amount of SP titles as well as online, and I'd count PC gamers using it to buy SP games customers, even though they won't show up that often using online stats.

So in total I suspect WW there are more people using Steam to buy, trial or play games and that as a result of the strength of its DD side it probably sells more (although it doesn't have the additional Live subscription fee so that probably evens things out a little vs Live).

That's incorrect. The second column is peak players today. Not peak overall. Means today there've been 450K people playing online... at 10AM... on a Wednesday... (and that's EST, most of the US is still earlier).

How on earth can Left 4 Dead have so extremely few players? I refuse to believe it.