There’s been a lot of talk about virtual currencies recently, with Facebook reportedly testing a payment system for third-party applications and MySpace considering something similar. Meanwhile, teen social network myYearbook is already making a healthy amount of cash with its virtual currency, dubbed “Lunch Money,” according to co-founder Catherine Cook. The New Hope, Pa., company’s annual revenue is in the eight figures (for the math-challenged: that’s tens of millions of dollars), and about a third of that comes from Lunch Money, she says.

Keep in mind that myYearbook only started charging for its virtual currency six months ago. Now users can earn Lunch Money by completing activities on the site or by buying it with real cash. They can then spend it on virtual gifts or use it to donate to charities of their choice in the myYearbook Causes program. (The site announced today that donated Lunch Money has saved 3 million square feet of rainforest, sent 22,000 books to Africa, and bought 40,000 pounds of rice for the hungry.)

Still, despite the cash Lunch Money is making for myYearbook and charities, the program is a bit more modest in its goals than the rumored virtual currency on Facebook. It won’t become the commerce system for a giant ecosystem of third-party applications, since all of myYearbook’s applications are in-house. And Cook says there are no immediate plans to change that.

Cook founded the company with her older brother Dave when she was 15, and the site has been growing fast. In fact, it was the only social network besides Facebook to grow in the U.S. last year , according to Hitwise — claiming that it has more than 10 million members. Cook says the company was also cash-flow positive for the first time in April. (Besides Lunch Money, the site earns revenue with advertising.) It has raised $17.1 million in venture funding .

I spoke to Cook, now 19, at the “Tech Titans of Tomorrow ” conference in Palo Alto, Calif. I’ve embedded a short video interview with her below, where she discusses founding the company and balancing her work with being a full-time college student at Georgetown. (Dave and Catherine’s older brother Geoff, a serial entrepreneur, is the company’s chief executive and its first investor, but Cook says she still puts in more than 50 hours a week on product development and flies home every weekend for work.)

Catherine Cook of myYearbook from Anthony Ha on Vimeo .