Catherine Cook: The brainwave that made me $10m by age 18

Three years ago, Catherine Cook – then 15 - and her brother – then 16 - had an idea for an online school yearbook. With help from their dotcom-entrepreneur brother, their company is now reaping $10m a year in ad sales.

Founded in March 2005 by Catherine and David Cook, has taken the social networking world by storm. In just three years, it has risen to third place in the sector, behind Facebook and MySpace. It's a feat all the more remarkable when you consider its two creators are only 18 and 19 respectively.

Three years ago, Catherine and David were flicking through their school yearbook the annual compendium of photos and memories that is a feature of most American high schools when they came across a picture of a girl they both knew. "This looks nothing like her," said David. Catherine agreed. It was clear, as she puts it, that "the picture stank". Wouldn't it, they wondered, be much better to do the yearbook online, and let people post their own photos?

The pair were lucky. Their older brother, Geoff, was already an established internet entrepreneur who had built up two successful websites, and, which offered online editing and writing services. In 2002, he had sold them for nearly $10m. When his brother and sister mentioned their idea for an online schoolbook, he immediately saw its potential to be "as big as Facebook for high school".

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The Cooks' parents thought the idea was crazy, but Geoff was undeterred. He offered $250,000 to get the yearbook project off of the ground, finding a web server to host it and a programming team in Mumbai, India, to turn it into a website for less than $25,000. It came complete with photo-sharing features and a place for students to outline their extra-curricular activities. Then, says Catherine, "it was guerrilla marketing all the way".

In April 2005, the pair began their campaign to get attention by wearing T-shirts to school emblazoned with the slogan, "Are you the prettiest girl in your high school? How about the dumbest? Find out at". It's the kind of question that would have your average British student cringing with embarrassment. But not in Montgomery High School, New Jersey. Within a week, 200 classmates had signed up. Users were offered a free T-shirt for every five people they recruited and within nine months 950,000 teenagers across America had flocked to the website.

This stoked up interest among companies such as Paramount Films, who began paying up to $500,000 to advertise on the website from July 2007. Suddenly was bringing in serious revenue. And David and Catherine haven't looked back since. This year, they're looking at $10m in advertising sales, having grown from three million users in November 2007 to ten million by July. But running a successful business while staying in full-time education hasn't always been an easy balancing act. "What ends up happening is that I just don't do my schoolwork," says Catherine, now at Georgetown University in Washington. "I go to classes but I never do the reading, which means I just cram it all in before the tests." Given the millions she's now making, her classmates are unlikely to feel too sorry for her.

Jody Clarke

Jody studied at the University of Limerick and she has been a senior writer for MoneyWeek for more than 15 years. Jody is experienced in interviewing, for example in her time she has dug into the lives of an ex-M15 agent and quirky business owners who have made millions. Jody’s other areas of expertise include advice on funds, stocks and house prices.