The rise of the freemium model has made differentiating App Store heroes from the also rans a trickier business.

No longer is publishing standard sales figures – a particularly popular move for many developers at this time of year – enough to determine an app's success. Tracking the revenue it's generated from such sales is the key question.

The Playforge, the developer of freemium release Zombie Farm, had much of its work done for it on this particular score, with the release of Apple's iTunes Rewind 2010 countdowns naming the studio's release as the top grossing freemium title in the US of 2010.

Farm wars

Coming in at number six on the countdown, Zombie Farm has been able to outpace other freemium titles, perhaps most notably Zynga's high-flying Farmville, throughout the course of year – although, on this case, it's worth noting Farmville didn't see the light of day until late June.

Nonetheless, Zombie Farm - released in February - has been downloaded 7.3 million times to date – the last 1.3 million within the last month – and can currently call on a monthly active userbase of 2 million.

In addition, The Playforge reveals that fans have clocked more than 46 million hours of gameplay and completed a total of 62 million brain-raiding invasions.

"Making it into Apple's top 10 grossing list this year is a colossal milestone for us," said Zombie Farm creator Vince McDonnell.

"It's inspiring, as we continue our commitment to constantly innovating with fun, highly functional and sometimes pretty ridiculous content for our players."

The numbers game

However, in regards to the game's exact revenue tally, Playforge isn't willing to play ball just yet.

Instead of dishing out cold, hard figures, the developer points to the likes of Doodle Jump, which it claims sold around 3 million units at 99c between January and June this year.

At this pace, it claims Doodle Jump will have amassed sales of around 7 million by the end of 2010, drawing in revenue of around $6.9 million before Apple's cut throughout the course of the year.

The fact that Doodle Jump finished 10th on Apple's top grossing list – four places below Zombie Farm – with what would appear to be a similar sales tally is, the studio suggests, evidence of the strength of the freemium model, and The Playforge's leadership of it.

Back in November, numbers published by GigaOM suggested between 30 and 40 percent of the top grossing apps on the App Store are freemium releases – a telling statistic, given Distimo estimates freemium releases account for just 1.34 percent of all titles on the marketplace.

You can also read our interview with The PlayForge's Vince McDonnell about the making of Zombie Farm here.