Although Valve hasn't publicly confirmed those figures, Newell has acknowledged that the company is "tremendously profitable" -- more so than Google and Apple per employee, in fact. Industry sources value the company at $2 to $4 billion, which is pretty incredible when you consider the fact that Valve only has 250 workers.
Steam's success is partially attributed to its appeal to publishers who earn a gross margin of around 70% on Steam compared to 30% via retail stores. Newell mentioned that the platform could easily be used to distribute films and music, but Valve is more focused on improving Steam for PC gamers than expanding to other content types.
As of last October, Valve announced that Steam witnessed a 178% on-year growth in new users, pushing the total number of active accounts beyond 30 million with more than six million members logging in each day. The store offered more than 1,200 games for download and sales grew over 200% from the previous year.