A few niche social networks are, unlike most of their peers, actually growing. MyYearbook claims to be one of them, with a youthful user base and emphasis on meeting new people.

MyYearbook has grown since its 2005 founding to 4.5 million unique visitors each month, according to CEO Geoff Cook, with 50 percent growth since last November. It also clocks an average of 167 minutes per user, per month. The new user growth may be driven by the tendency of teenagers to look for a place of their own (in this case, besides Facebook), but the high engagement is driven by the site’s games.

When it comes to the games, MyYearbook is again a bit different. For two years, casual gaming company Arkadium has been behind MyYearbook’s in-house selection of games, with relatively little involvement from other companies.

Now, however, MyYearbook is opening up to outside developers, starting with Omgpop, which runs its own teen-dominated platform. Omgpop will get its own tab on MyYearbook’s site, with integration of games like Balloono and the Pictionary-like Draw My Thing.

Omgpop’s games will also feed off MyYearbook’s in-house virtual currency, called Lunch Money. Unlike Facebook’s Credits, Lunch Money is given out in large quantities as a free promotional tool.

Lunch Money is easy to get, but it’s also won, lost and otherwise used all over the site, so sales of additional Lunch Money end up driving about a third of MyYearbook’s total monetization; Cook says the revenue run rate is up to about $24 million a year, and has grown 64 percent since the start of the year.

Any developers adding their games to MyYearbook’s site will also end up working with Lunch Money, which can be converted into other currencies, like Omgpop’s Coins. That works well for the latter company, since it has a similar model, giving out some Coins for free.

Going forward, sites like MyYearbook could drive an interesting alternative to Facebook’s social gaming ecosystem. On MyYearbook, games are an integral part of the experience; some even feature in the site-wide news feed, which is called Chatter.

Also, though it hardly seems possible, MyYearbook’s games are generally faster and lighter experiences than your typical Facebook farming or city building game, with a heavier social interaction component.

Part of the difference is that MyYearbook and Omgpop both promote interaction between unassociated users. “People have a desire to play not just with friends, but based on level, genre, skills, and to meet new people,” says Omgpop chief revenue officer Wilson Kriegel.

That means more synchronous gaming, but with the same social networking features that made Facebook’s games more successful than their casual counterparts.