In its monthly sales estimates, the NPD Group only offers figures of physical retail sales of non-PC games, not digital downloads of Windows-based titles. Today, though, the US industry-research firm offered estimates for digital downloads for last year…and the results won't terribly astonish anyone familiar with PC gaming.

　　Blizzard's line of World of Warcraft credit is still open…very open, in fact.

　　For the 12 months ending December 31, 2009, some 21.3 million games were bought digitally in the US--roughly in line with the 23.5 million units sold at retail during the same year. Valve Software's Steam was the top online retailer, followed by Direct2drive, Blizzard.com, EA.com, and Worldofwarcraft.com--another extension of Blizzard's massive massively multiplayer empire.

　　Top Five Frontline Digital Retailers –2009 (based on unit % share)

　　1. Steampowered.com

　　2. Direct2drive.com

　　3. Blizzard.com

　　4. EA.com

　　5. Worldofwarcraft.com

　　Casual and social network gaming was also a big draw, said NPD analyst Anita Frazier. "The popularity of social network gaming increased from Q3’09 to Q4’09 as 4.8 million more people played games on a social network in the U.S.," she explained. "This demonstrates how consumers can now experience casual types of games through myriad vehicles, broadening the competitive landscape."

　　NPD also provided the list of top social and casual game designers, which was led by Seattle-based Big Fish Games. EA-owned Pogo.com was second, with Gamehouse.com in third, iWin.com in fourth, and Realarcade.com in fifth.

　　Top Five Casual Digital Retailers – 2009 (based on unit % share)

　　1. Bigfishgames.com

　　2. Pogo.com

　　3. Gamehouse.com

　　4. iWin.com

　　5. Realarcade.com

　　The NPD survey was based on responses from 8,000 consumers, and the resulting data is "weighted and projected to be representative of the U.S. population ages 2 and older." The industry research firm's normal data comes from a panel of over 180,000 individuals, whose habits are tracked via weekly studies.