Not long after Catherine Cook arrived at her new high school aged 15, she was flipping through an old yearbook with Dave, her 16-year-old brother, when they spotted the name of a girl they both knew. Yet the yearbook picture looked nothing like her. In that moment an internet empire was born.

You have heard of Google, MySpace, Facebook and the geeky boy geniuses who made billions from brilliant computer ideas. Now meet a geeky girl genius - a New Jersey teenager who turned a stroke of high school inspiration into what is now the fastest-growing online social network in America.

Three years after it occurred to Cook that one of icons of the American educational experience - the high school yearbook - was badly in need of updating, she is sitting on an internet fortune.

The website she launched with her brother at Montgomery high school in Skillman, New Jersey, has exploded into a multi-million-dollar online business, with 10m members of myYearbook.com registering 1.5 billion page views a month.

As the site’s co-founder and features guru, Cook, now 18, presides over a roster of online activities that have expanded the yearbook idea into a games, video and quiz-packed site with a heavy accent on the joys and perils of dating. Popular quiz topics include: “Do boys like you? What kind of kisser are you? What’s your break-up style?”

Launched in 2005 with an initial investment of $250,000 (£135,000), the site is now earning more than $10m a year in advertising sales and Cook has become a millionaire.

Hitwise, the leading internet monitoring group, has calculated that in June myYearbook became the third largest online social network in the United States, behind MySpace and Facebook, both of which are intended for older users.

“We’ve had some offers from people who want to buy us out - and a few pretty big ones,” Cook said last week.

“But we’re not interested right now. I’m having too much fun working on the site.”

With their joky style - the website declares that its “vice-president for marketing” is Dusty, the family dog - Cook and her brother may have dealt a death blow to the rather dreary school annuals that in recent years have been read mainly by magazine editors seeking embarrassing pictures of celebrities.

“I realised when I was talking to Dave that those old yearbooks really sucked,” said Cook, who wears big glasses and admits to being “a bit nerdish”.

She added: “The pictures always stink and the books never really tell you anything about the person. So we thought: why not put it online? Why not have a profile page that can list your TV interests, the music you like, your extracurriculars?”

Neither Cook nor her brother knew anything about building a website, but family help was at hand. Catherine and Dave have an older brother, Geoff, who had already made an internet fortune of his own while studying at Harvard University.

Geoff had devised and sold a pair of websites that helped students with essay writing and preparing CVs. He immediately saw the promise in his younger siblings’ idea. He wrote them a cheque to get the website off the ground and has since become the venture’s chief executive.

“Geoff is 11 years older than me,” said Cook. “He was running his websites out of Palm Springs, California, and my mom used to send us out there to stay with him. His office was just so cool and I knew when I saw it that I didn’t want a proper, typical job. I wanted to be an entrepreneur.”

At 15, when most of her schoolmates were shopping at the mall and worrying about boys, Cook was firing off e-mails to a team of website designers in Mumbai.

“It was very helpful to have a close personal relationship with someone like Geoff,” she said.

“He knows how to hire programmers in India, lease server space in Texas and start conversations with venture capitalists in Silicon Valley.”

By April 2005 the family was ready to test its new toy. She and Dave turned up at school wearing T-shirts with slogans that read: “Are you the prettiest girl in school? How about the dumbest? Find out at myYearbook.com.”

As teenagers around the country flocked to the site, they were followed by a stampede of investors and advertisers. The site now employs 70 people and Cook and her brothers may eventually be worth hundreds of millions of dollars. Facebook, for example, which is privately controlled, is estimated to be worth between $3 billion and $15 billion.

Cook is about to start her second year as a student at Georgetown University in Washington DC, but she acknowledged that she was finding it hard to combine her studies with running a budding internet conglomerate.

“I do want to graduate,” she said. “But I may have to take one or two years off to work on the website first.”

Times Recommends

• Robert McNamara dies aged 93
• German soldiers earn new ‘Iron Cross’
• Great Barrier Reef 'gone within 20 years'