Part of the detail of Rovio's surprising $42 million investment round, comes news that the various games in the Angry Birds' family are now generating 40 million monthly active users.

It's a significant figure for the company to reveal, not least because it's previously focused on the number of downloads of the game.

Retain your appeal

Monthly Active User (MAU) is a term more often used by the operators of freemium games, both on Facebook and mobile, because it better describes a game's audience in terms of how many people are regularly playing, and hence how many are likely to be spending cash on in-app purchases.

For example, with 10 million downloads of its Smurfs' Village and Zombie Cafe games, Capcom Mobile now has 6.4 million MAUs, Zynga with Friends' Words with Friends has around five million MAUs, while mobile social giant's DeNA and ngmoco are planning to reach 100 million MAUs.

Hybrid model

Despite the introduction of in-app purchases such as the 99c smartbomb Mighty Eagle, which Rovio says is being used by 40 percent of "new users", Angry Birds isn't a traditional freemium game however as it doesn't have the option to buy in-game currency.

Yet, with the company saying it will use part of its investment to roll out "an online Angry Birds experience" - presumably Facebook, web and mobile - it seems it's switching its terms of success to a more service-driven approach.

Indeed, as ngmoco's CEO Neil Young pointed out at GDC 2011 - in reference to its switch from download numbers to active players - "We become what we measure."