Digital distribution accounted for 48% of PC games sold in the USA last year, according to a new report from market research firm The NPD Group.

21.3 million "PC Game full-game digital downloads" were purchased in the USA in 2009 while 23.5 million physical units were bought at retail during the year, according to NPD's figures. DD raked in 48% of unit sales and 36% of dollar sales.

The group also offers its own ranking of the various digital distributors, divided into categories of "casual digital retailers" and those "which often focus on titles that are also offered in retail stores as physical purchases." The ranking supports Direct2Drive's refutation of Impulse's claim that it held second place in the PC DD market, at least.

Top 5 Frontline Digital Retailers - 2009 (based on unit % share)

1. Steampowered.com
2. Direct2Drive.com
3. Blizzard.com
4. EA.com
5. Worldofwarcraft.com

Top 5 Casual Digital Retailers - 2009 (based on unit % share)

1. Bigfishgames.com
2. Pogo.com
3. Gamehouse.com
4. iWin.com
5. Realarcade.com

NPD notes that the market share of "Casual Digital Retailers" declined in 2009 over the previous year, which the group speculates is in part due to "the increase in popularity of free social network gaming and free mobile gaming."

"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.," said NPD analyst Anita Frazier. "This demonstrates how consumers can now experience casual types of games through myriad vehicles, broadening the competitive landscape."

Two separate surveys served as the foundation for the report--a quarterly survey of 8,000 members of its "online consumer panel" and a weekly survey of 180,000 members.