Market research firm The NPD Group has released a new PC Games Digital Downloads analysis report, which finds that 21.3 million PC game full-game digital downloads were purchased online in the US last year, compared to 23.5 million purchased at retail during the same period, indicating that digital downloads are nearing parity with in-store purchases.

NPD research indicates that PC physical retail sales experienced a year-over-year revenue decline in 2009, but sales were still slightly larger than PC digital download sales on a unit basis. PC digital downloads represented nearly half of unit sales across digital and retail at 48% in 2009, and accounted for 36% of dollar sales.

The report divides the PC full-game digital download landscape into two web-based retailer segments: Casual Digital Retailers, which mainly focus on smaller, easily accessible games that typically utilize try-and-buy or advertising revenue models, and Frontline Digital Retailers, which focuses on titles that are also offered in retail stores as physical purchases. The latter increased its share of the PC full-game digital download market in the second half of 2009, at the expense of the Casual Digital Retailers.

One of the most significant factors contributing to the decline in share captured by Casual Digital Retailers is the rise of free social network gaming and free mobile gaming, which have undoubtedly broadened the competitive landscape.

Free mobile gaming has captured the attention of gamers, shown by the 30% increase in usage of the iPhone and the iPod Touch as gaming devices from Q2 2009 to Q4 2009. Additionally, 97% of those who downloaded a game app in Q4’09 downloaded a free version of a game.