We know that Steam's on the grow, the five million concurrent users earlier this week indicated as much. Valve have released some more info to prove that their digital distribution service is still expanding at an impressive clip.
Sales have grown by 100% for the seventh year in a row in 2011. There are now 1800 games on Steam. 40 million people have registered for accounts (though not all will be active), and 780 Petabytes of data has been distributed to customers over the course of the year. 19 million items have also been traded. Valve don't mention how many of those were hats.
Valve's Steamworks service for publishers and developers is also gaining plenty of traction. Steamworks provides secure DRM, cloud saving, statistical feedback and achievement support, and is now used by 400 of Steam's 1800 games. 14.5 million Steamworks games have been shifted in the past year, 67% more than in 2010.
If Steam keeps on growing at this rate, it will have consumed the known universe by 2031. Before that happens, Valve are planning to release a Big Picture UI mode designed to fit Steam to large screens.