I think it's amazing that a single service on PC could grow a costumer base so big. I think it's possible that Steam may now have more gamers than PS3, or even X360.

In the beginning of last year Valve reported Steam had over 20 millions costumers. Then, in 2009, several games released their retail copies with Steam-integration. Among which were:

1. Empire: Total War
2. Dawn of War 2
3. Modern Warfare 2
4. Left 4 Dead 2
5. The Last Remnant
6. Stormrise
7. Silent Hill Homecoming
8. Football Manager 2010
9. F.E.A.R. 2: Project Origin
10. Saint's Row 2

I think Steam has a really good chance of having more costumers than PS3. Not so much against X360 yet.

i think it does, some of my friends have ps3s whereas a vast majority of my friends have a steam account and use it regularly

It would be great to know a couple of things about steam... unfortunately we get no such information. Things I would like to know includes:

Userbase
Active userbase (I have friends that haven't logged in to steam in YEARS)
Attach rate
Yearly revenue
Individual game sales.

But unless we actually get answers on at least one of those I cannot speculate as to how many people are using Steam. I doubt it's more than 30 million anyway, so I'd put it behind PS3 and Xbox.

This doesn't compeltely answer your questions, but:

In Feb 2009, Valve reported more than 20 million active steam subscribers. I'd assume this means they log on at least once a week, as I believe that's the same qualification Sony and MS use when they report active subscribers. Source: http://g4tv.com/thefeed/blog/post/693342/Live-Blog-DICE-2009-Keynote---Gabe-Newell-Valve-Software.html

Xbox Live just reported only 20 million active active subscribers last week, which is all accounts including their gold, silver, windows live, and even forum accounts (I had an xbox live account for a full year before I got a 360). Source: http://www.kotaku.com.au/2010/01/xbox-360-39-million-consoles-sold-only-half-are-on-xbox-live/

In that same report, Microsoft bragged that they reached a record of 2.2 million users online simultaneously. Steam however far exedes that number on a daily basis, and its peak number of congruent members is almost 2.5 million daily. What their peak is I have no idea, but I would assume it's much much larger. Source: http://store.steampowered.com/stats/

As far as revenue, Steam doesn't release such information, but I do know World of Goo, despite being available in many locations, made the mast majority of its money on Steam. Source: http://www.offworld.com/2009/03/indie-games-summit-ron-carmel.html

I don't think there's any question that Steam is a signifficantly larger online service than Xbox Live and PSN. The only remaining question is if it's larger than them combined... but that seems unlikely.

I know a Steam moderator/admin/employee once said that an active costumer means a costumer with a game associated to it's account.

In other words, in early 2009 there were already over 20 millions people who have bought a Steam game (including retail copies).

costumers as in wearing costumes?

I'll wait here untill Shio comes with his calculations... :)

As a source for digital distribution, Steam no doubt is bigger than either Live or PSN.

Costumers = cosplayers?