Many have speculated over the past few years that digital downloads of PC games would inevitably surpass their retail counterparts. Thanks to a recent study conducted by the NPD Group, it would seem as though those speculators finally have some data to back that assumption up.

The report reveals that the number of PC games sold via digital download fell just shy of reaching parity with their brick-and-mortar counterparts. 21.3 million PC games were downloaded online via sites like Big Fish Games while 23.5 million were purchased in real world retail stores. NPD also points out that retail sales of PC games took a sharp decline last year, declining to $538 million in 2009 – a 28% drop from their numbers in 2008.

This may sound like quite the coup for casual games, an industry largely centered around digital content delivery. However with a growing spectrum of options available to casual players, ranging from iPhone to Facebook, the casual PC downloads market has actually taken something of a hit.

“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 game analyst Anita Frazier. “This demonstrates how consumers can now experience casual types of games through myriad vehicles, broadening the competitive landscape.”

The report further goes on to estimate ranks for top digital retailers in two categories, frontline (online services that sell games you can also buy in stores) and casual;

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

While it’s nice to see the estimated rankings, it’s hard to really interpret them without seeing the sales figures used to calculate them. How well does the casual sector perform compared to frontline, for example? And how big of a lead does Big Fish Games have over Pogo?

With the launch of cloud-gaming services like OnLive potentially bringing the number digital downloads up, and the further fragmenting of the casual games market potentially bringing those same numbers down, 2010 could turn out to be a roller coaster year for the PC downloads market. It should be interesting to see how these numbers look if NPD follows this up with a similar report a year from now.