According to the Half-Life and Left 4 Dead dev, this "extraordinary growth" marks "the fifth straight year the platform has realized over 100% year-over-year growth in unit sales."

"Active accounts" also rose 25% to pass 25 million, of which ten million have Steam Community profiles--though Valve does not explain what constitutes an "active" account.

"The peak number of concurrent users eclipsed the 2.5 million mark during the month of December," says Valve--no doubt helped by the fact that Infinity Ward's monster hit Call of Duty: Modern Warfare 2 requires Steam. This has helped Steam's "average monthly player minutes" reach a monster thirteen billion.

"Steam turned five years old in March 2009," said Valve president Gabe Newell. "With the introduction of each new platform feature released over the years--such as the Steam Community, Steam Cloud, and Steamworks--we've seen corresponding growth in account numbers, concurrent player numbers and developer support for the platform. As such, we plan to continue to expand and grow the platform to better serve the developers supporting the open platform and millions of gamers logging in each day."

Newell was recently named as recipient of this year's Game Developers Choice Pioneer Award in part for his role in co-creating Steam, which "has become a key way for many smaller and larger PC game developers to gain fans and make money without requiring a physical retail publisher" according to the organiser Think Services Game Group.

The impressive unit sales figures will of course have been helped dramatically by Steam's regular 'Midweek Madness' and 'Weekend Deal' sales which see select titles discounted heavily, not forgetting its colossal holiday sales.

The PC digital distribution market continues to grow increasingly crowded. Microsoft recently joined in the action with its Games for Windows Live Games on Demand service, competing directly with the platforms of Steam, Impulse, Direct2Drive and such.

Indie development and modding hub ModDB's parent company Desura is launching a platform of its own, focused on selling mods and indie games, while Green Man Gaming will seek to stand out from the crowd by offering trade-ins on its games.

The busy market has lead to a fair amount of jostling between. Developer and publisher Stardock's recent estimate that it held the second-largest slice of the digital distribution pie after Steam was contended quickly and loudly by rivals GamersGate and Direct2Drive.

Steam has also been the target of criticism from both competitors and developers. In October, 2009 Gearbox CEO Randy Pitchford accused Valve of "exploiting a lot of small guys" by "taking a larger share than it should for the service it's providing."

However these claims were rebuked by others in the industry including notable indie developers behind such titles as World of Goo, Machinarium and Audiosurf as well as Tales of Monkey Island developer Telltale Games, all of whom have games on Steam.

Several rival digital distributors have taken issue with Valve's Steamworks suite--which offers a DRM solution, community features and auto-updating, among other features--claiming it makes effective exclusives of games as they do not wish to sell games which bundle a rival's distribution platform.

Modern Warfare 2 proved to be the flashpoint of these concerns, when Direct2Drive, GamersGate and Impulse all refused to stock the game due to its use of Steamworks--though Impulse carry no other titles published by Activision anyway. D2D famously referred to MW2's use of Steamworks as a "Trojan Horse" for Valve's platform.