Main reasons I can think of:

- Steam get royalties on every game downloaded, which is the major selling point of the service on PC. Often, Valve is the primary distribution model for small indie devs.
- As far as I'm aware, Valve have a relatively small team compared to Sony and Microsoft's teams for PSN and X-box Live.
- Valve have no hardware R&D costs and concentrate solely on software and network facilities.
- A lot of Valve games seem to have long legs and sell for a very longtime on Steam (just see the regular Steam chart).

Steam owns over 150 Thousand Servers...

I recognize it's still a ridiculous number of servers.  However, the OP made it seem like the servers were used for gameplay itself.  This is not true, from what I understand. 

 I think they are probably around break even or a little better. They are very much in expansion mode at present in terms of getting the number of servers into the ground to support a growing online network. This costs a lot of money but you have to balance it out with Xbox Live and PSN where the average Steam user probably spends in the order of 2-3* as much as a typical PSN/XBL user considering they often pay a full price $50-60 for their games and many people buy all/most of their titles on Steam. I think the fact that the downloads aren't as time sensitive as say a movie or XBL download significantly shelves their expenses.

The only reason why L4D sold more on PC than 360 is because it was sold at a discount. I hardly doubt it sold alot on PC at the $50 price tag.

They dont own all those servers. They own the base Steam servers and probably some CS/HL/TF servers. All other servers are using their software to appear on the server browser.

I don't even know where the XBox Live money goes, since customers who paid for Halo 2 and then paid for XBox Live for several years can't play Halo 2 anymore. What's the point if the fees don't actually support the games you paid for?

I paid for StarCraft once in 1998, and I got at least 12 free years of online multiplayer.

In January 2010 Valve announced that it had surpassed 25 million active user accounts. Those numbers with some ads rack up some nice cash. According to Wiki they begun being profitable in 2005 and can get $30 profit from a $50 game, as retail only receives $7.50.

obviously most of there income comes from the steam store and they get a take of every game sold and if a game is discounted then it's is up to the games publisher to make the call and wouldn't affect valves bottom end ,plus a lot of the big sellers are there own games and a lot of them because of there age would have recouped there money years ago and all steam sales would be pure profit on valves older games, plus they have a lot more games from different publishers catalogues then live and psn

How do you know L4D sold more on PC than in X360?