It's hard not to wonder just what the Cook parents fed their kids, Geoff, Dave, and Catherine. Geoff founded a business while attending Harvard—and sold it in 2002 at the ripe age of 24. That inspired Catherine and Dave to launch myYearbook, a social networking site for high school kids, in 2005. When the site, based in New Hope, Pennsylvania, took off, Geoff stepped in as CEO. (Dave spent the past four years at college in Colorado but recently returned.) Last year, myYearbook, one of the nation's 25 most-trafficked websites, merged with Quepasa, a publicly traded company that runs social networking sites aimed at Latinos, in a $100 million deal. The Cooks still run the show and are focused on graduating to a global market. As told to Liz Welch.

CATHERINE: When I was 14, my family moved to Skillman, New Jersey, where I started high school. I didn't know anyone. My brother Dave and I were flipping through the school's yearbook to see if we recognized anyone, and we were shocked at how useless the information was. That's when we thought, What if we put the whole thing online? I'd always looked up to my brother Geoff. He's 11 years older than me and had already launched a successful start-up. I thought if he started a company, I could, too.

GEOFF: I was a sophomore at Harvard and was editing admissions essays and resumés. In 1997, I put up two websites— and By the end of my senior year, I was hiring students to do the work and making hundreds of thousands of dollars. I wound up finding an investor, and then sold the sites to the Thomson Corporation in 2002. I stayed with Thomson—which had offices in New Jersey, not far from where my family lived. I was looking for my Next Big Thing when Catherine bounced this idea off of me. I invested $250,000 and left Thomson and became myYearbook's CEO.

CATHERINE: I was like, Wow! He really likes it! We found developers in Mumbai through a Google search. Dave and I would get home from school at 4 and then be up chatting online until 4 a.m. because of the time difference. A month later, in April 2005, we launched a bare-bones site in our town. We wore T-shirts with sayings like, "Are you the prettiest girl in high school? How about the dumbest? Find out.'' In one week, 400 people signed up. I knew we were going to be successful when a friend told me it was one of the greatest procrastination tools ever built.

GEOFF: We immediately started thinking of ways to engage users with games and quizzes that appeal to teenagers. Catherine was responsible for some of our most important features, because she was communicating directly with users and learning instantly what they wanted.

CATHERINE: I have always been, and still am, one of the most active members on the site. Every new member gets a friend request from me, which is why I now have more than a million friends.

GEOFF: By mid-2006, we had one million users, mostly due to viral growth, but we weren't making any money at all. I reached out to one of the investors in my previous company, and he made a sizable investment, which allowed us to put together a team of about a dozen people—mostly engineers—and get our first office. Before that, we were working out of my home and my parents' place. But then we realized that we needed more engineers to scale the site, so we started looking for a second round of financing. Our selling pitch was that we were MySpace for high school. We got $4.1 million in 2007.

CATHERINE: Dave and I became known as the "myYearbook kids" at school. Our plan worked-we had tons of friends. But my grades suffered. By my senior year, I was skipping classes all the time to go to work. One of my teachers actually pulled me aside, because my grade had dropped to a 78 percent-and I had always been an A student. I missed one class so often I almost lost credit, but my school cut me slack.

GEOFF: Before we launched the company, Catherine cared about if she got an A-minus instead of an A-plus. Her striving for perfection transferred to the company. She became our spokesperson, and she's a natural. When she was 17, we were invited to speak at a technology conference with all these very high-powered people, including Mark Zuckerberg. We had to pitch our business to Barry Diller for a segment called "What Would Barry Buy?" Catherine and I followed MC Hammer, who presented his company, DanceJam. Barry picked myYearbook.

CATHERINE: We weren't making any money, so after I graduated, I went to Georgetown as a fallback. Dave wanted to focus on college, so he took time off from the company while he was at the University of Colorado. I came home every two weeks and clustered all my classes to two days and spent the other three telecommuting. As a result, I had no social life. There was no time for parties or extracurricular stuff. I felt bad about missing things. But how many kids could say, I'm missing my first midterm to talk about user-generated content at a conference in South Korea?

GEOFF: By the time Catherine was at college, it became clear that Facebook intended to rule the world, which forced us to hone our message: Facebook was about the friends you already have; myYearbook was about making new friends. On Facebook, people never accept friend requests from someone they don't know, but that is 99 percent of all friend requests on myYearbook. So we focused all our apps and features on becoming the social graph of people you want to know.

CATHERINE: I met my best friend on the site when I was in high school. Recently, a member messaged me to say his wife, whom he met on myYearbook, was pregnant with their first child. How cool is that?

GEOFF: In 2010, we had $23 million in revenue, but 85 percent of our users were in North America. Winners tend to be global brands, so we started looking for ways that myYearbook could span the world. We looked at raising private equity to go buy other companies, but we didn't see anything we were interested in. And then, in 2011, we were approached by Quepasa. We thought, If you put these things together and push, a year from now we'll have a meaningful Brazilian and Mexican audience. 

CATHERINE: The merger made me even more excited to graduate. I kept it a big secret. I didn't even tell my boyfriend or best friend.

GEOFF: It was a $100 million deal—$18 million in cash, the rest in stock. I am now Quepasa's COO. Since yearbooks don't matter much outside of the U.S., we just announced a new name: MeetMe. And with that, we plan to make our social discovery platform span the world.

CATHERINE: My new title is vice president of brand strategy, so I'm focused on going global with the mission I started this company with: to meet new people. It's pretty awesome. I'm more popular than I ever imagined.