IMO in case of buyout Google is most likely buyer. EA cant afford it (Valve is most likely worth much more than EA - EA has market cap. "only" 5.33 Billion USD). MS looks to be more interested in consoles. Activision/Blizzard is also likely able to put together enough money, but if Gabe offered Valve for sale I think that Google would get it.
Reason why I dont think buyout is likely to happen anytime soon (next 5 years) is that Gabe already have tons of money and no reason to sell.
I agree that it would be great to know financial results of Valve (Steam). I wrote article about digital distribution services few months ago and it was wery difficult to make reasonable estimate of marketshare (several Steams competitors are also private companies).

They had $8.24 billion IN PROFIT last year