http://www.shacknews.com/onearticle.x/62158

Steam's unit sales increased by 205% in 2009 over 2008's figures, Valve has declared as part of an announcement detailing the digital distribution platform's growth last year.

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."
Keep putting up extraordinary weekend deals, and I'll keep buying.