"Angry Birds watch out," the headline reads on a story from PaidContent, because "Club Penguin Puffles is launching on an iPhone near you." As the article notes, while Angry Birds and other social games are incredibly popular on mobile devices (see Alexis Madrigal's calculations about just how devastatingly popular), there have been few wildly successful, "addicting" mobile games aimed directly at children. But with the arrival of Club Penguin on the iPhone, it's pretty clear that companies are seriously taking aim at that young consumer/mobile market and hoping they can create "the next big thing."

Club Penguin has long been a "big thing" on the Web. It is one of the largest social networks for kids. Launched in 2005, it was acquired by Disney in 2007 for over $300 million and boasts about 12 million users. It's a destination site for many kids in the 6 to 14 year old range, who go there to play games, to outfit their penguin avatars with various virtual goods, and to chat with one another. According to data from Google DoubleClick and the Financial Times, the average user visits Club Penguin 3.8 times per month, spending between 12 and 54 minutes on the site.

Those sorts of engagement numbers are a metric that's closely monitored by websites (and not just those aimed at kids, of course).

Now I won't argue that "Club Penguin is a waste of time" or that "Angry Birds is a waste of time." But even though some people have found that Angry Birds can offer some useful physics lessons, I think it's pretty easy to categorize a lot of these sorts of games as sheer entertainment and not all that educational.

How then do we get that same level of engagement, that same monthly active user rate, in a specifically educational social gaming site?

That's something that a gaming site called BrainNook is working on, and the company behind it is already boasting numbers that it claims demonstrate it's able to keep kids as active and interested as the popular entertainment websites like Club Penguin and Moshi Monsters. BrainNook sees users return 3.3 times per month and spend an average of 28 minutes on the site. Moshi Monsters, to offer some more figures for comparison, sees 3.4 repeat visits per month, and users' sessions average 13 minutes.

Part of the success of BrainNook is getting kids to stay and play may well be the background of founder and CEO Abhi Vijayakar. Before starting Nunook, the parent company behind BrainNook, Vijayakar worked for EA Games on projects that included the popular Sims games. He said he was fascinated by the ways in which kids in particular interacted with Sims "without thinking of them as educational games."

He's taken that background in engaging and exploratory gameplay and applied it to BrainNook, a social gaming site that teaches both math and language skills. Aimed at grades 1 through 5 and designed for use at school and at home, the games are tied to the Common Core State Standards.

BrainNook also offers performance reports for parents or teachers, and Vijayakar says the company wants to be able to allow teachers to generate assignments via BrainNook too, so that teachers can put data in in addition to be able to get data out about kids' activities.

While the adoption of Common Core might appeal to educators and parents, it's not the thing that keep kids coming back. Those are, in many ways, the same things that make Moshi Monsters and Club Penguin appealing: you can customize your avatar; you can chat (with some restrictions, of course) with friends; you earn points and badges in games that are both colorful and fairly typical for Flash-based gaming.

Vijayakar argues that what's behind the high rate of engagement for BrainNook however isn't the "fun" in these individual games per se but rather the over-arching narrative that runs through the BrainNook site (in this case, it's the classic "help the lost alien get home" story). That's a piece that, I'd argue, is missing from a lot of the educational games I see: how do you motivate kids to keep playing/practicing? Why do kids want to play? Well, having a major storyline is one way games encourage that engagement. The bigger mission, whether rescue the princess or save Azeroth, is what makes you feel as though you're participating in something, well, big. Something "epic," to use Jane McGonigal's terminology.

BrainNook is a recent graduate from the Imagine K12 startup incubator. The site is free to join, amd like similar offerings, offers premium features for paid subscribers. But unlike the new Club Penguin iOS app, there isn't yet a mobile version of BrainNook.