Quick – name a first-person shooter game that you can play on Facebook.

If Uberstrike didn’t immediately come to mind, you must not be one of the 5 million gamers playing it. The free-to-play and cross-platform game is doing crazy numbers and is set to explode on the iPad and Android tablets. It has more than 800,000 Likes on Facebook, was downloaded 1 million times through the Mac App Store (amounting to 1 percent of all downloads), and has 1 million monthly active users.

Oh, and it has just announced first-round funding from Atomico, the venture capital firm started by Skype co-founder Niklas Zennström. Cmune wouldn’t disclose a dollar figure for its funding round, but it is likely in the low millions. Competitor Kixeye last year raised $18 million in its Series C round, for example, and had only spent $4.5 million until that point.

So who’s behind Uberstrike? A 3D social games developer called Cmune, based out of Beijing. The small company is an example of a handful of China-based Internet players that are targeting foreign markets, including Dolphin (mobile browser), Papaya Mobile (Android games), Lakoo (mobile games), AppAnnie (app analytics), and Happy Latte (mobile games).

It also sits in the tier below China Internet giant Tencent, which makes much of its money from gaming and is eyeing the hardcore gaming market in the US with its successful free-to-play model. Tencent hinted at its US strategy when it last year acquired Riot Games. Like Tencent, Cmune makes its money in Uberstrike by selling virtual items, such as guns and armor, to players who want to enhance their playing experience.

Uberstrike is unusual in that it is a hardcore game that exists within a space, Facebook, that has traditionally been more fertile territory for casual social games, like those produced by Zynga. As Gameloft and 3G Studios have found – their hardcore Facebook games NOVA and Brave Arms both closed in 2011 – it’s tough to foster a gaming community on Facebook outside of Farmville and friends. But those failures have also opened up opportunities. There are few options for fast-paced, exciting skill-based games on social platforms, meaning Cmune can capitalize by introducing new types of gamers to the form.

That much is shown in Uberstrike’s demographics. Cmune founding partner Benjamin Joffe told me that 85 percent of the game’s players are male, but they range in age from 10 years old to older than 60. The game enjoys one of the highest average revenues per user on Facebook (although Joffe wouldn’t disclose the figure), and more than $20 per month from paying users who came to the game via the Mac app store. The figure is lower for Facebook, because the barrier to entry is lower – essentially, lots of kids play the game in internet cafes, while Mac users are probably higher-income because they can at least afford expensive Apple products.

Despite the technical challenges of building on top of a social network, Cmune has grown Uberstrike as a major community in its own right, harnessing the volunteer manpower of some of its most avid players and employing a community manager. It has also pulled off a significant technical feat by also making the game available through a website, in the Mac app store, and soon on tablets, allowing seamless multiplayer interaction between platforms and devices. So far the games reaches mostly a fairly young demographic – with teenagers in the dominant demographic bracket – but as the game becomes available to the more moneyed iPad crowd, it can probably expect a significant bump in revenues.

Just last week, Cmune also announced a partnership with GameStop’s browser-based games channel Kongregate, giving it yet another powerful distribution channel.

While Cmune is based in China – and, significantly, started in the advanced gaming market of South Korea – it is international in make-up, drawing on talent from around the world for its 20-person team. Funnily enough, it hasn’t launched Uberstrike in China. That will change soon.