Every evening, Juliette Brindak writes a to-do list of gargantuan proportions. It ticks down, hour-by-hour, and sometimes minute-by minute, what she needs to do the next day, and exactly where she needs to be. That includes classes for her double-major in anthropology and public health at Washington University in St. Louis, homework assignments, and social time. It also includes meetings with investors, calls with employees, and moderating discussion boards on which 12-year-old girls debate the aesthetic merits of the three Jonas brothers.

That's because the college senior is also CEO of a website for pre-teen and tween girls that's been valued at $15 million and is ranked the third "girls only" website worldwide, based on Alexa data. It's called MissOandFriends. If you've never heard of it, you're probably not an American girl between the ages of 8 and 14. You probably aren't a fan of online quizzes about Miley Cyrus, or virtual dress-up games, or message boards centered around how to talk to boys.

Before the website MissOandFriends existed, there was Miss O, a character drawn by Juliette to amuse her little sister, Olivia. For Olivia's eighth birthday party, Juliette illustrated images of all of Olivia's friends. "They just went crazy for them," Brindak says of the girls. Juliette and Olivia's mother, a designer, digitized the illustrations, and their father, who had a background in business, started helping the girls think about making a business out of content by girls, for girls. The site launched in April 2005, and now has significant investment from Proctor & Gamble.

The most popular parts of the website are the message boards, which are completely adult-moderated to be compliant with the Childrens Online Privacy Act, and the games. "Girls love dress-up games. They love to dress up their favorite celebrities. We've asked the girls what kinds of games they want, and are constantly changing the content," Brindak says. "We are by girls for girls, so we always ask them what they want, and they tell us."

MissOandFriends, the company, is based in Old Greenwich, Connecticut, with more than a dozen employees scattered throughout the United States and Canada, mostly in New York, Texas, and California.

"We're definitely a virtual company. We really say there are no politics within the company, and there's not that water cooler phenomenon, because we are virtual," Brindak says. That said, hiring has been one of her most significant challenges.

"We've brought on a couple people with good backgrounds who seem incredible and talk a big game, but then nothing happens," she says. "It's crazy that that sort of thing happens. Maybe they really don't have that experience. It's been disappointing in many cases."

But Brindak says she's learned how to read people—and to have the confidence to both make judgements and allow herself to stand up to judgement by investors and partners.

"Being able to present my business to big companies has built my own confidence and self-esteem," she says. "It's incredible that the company's aim—to instill confidence in girls—has actually been something that's happened simultaneously to me."