NEW YORK — It has been 10 years since a Harvard sophomore named Mark Zuckerberg created a website called Thefacebook.com to let his classmates find their friends online.
They did. And in the decade since, so have more than a billion people, not just American college students but also farmers in India, activists in Egypt and pop stars in South Korea.
Facebook has transformed how much of the world communicates. Zuckerberg's insistence that people use real identities, not quirky screennames, helped blur, if not erase entirely, the divide between our online and offline worlds. Long-lost friends are no longer lost — they are on Facebook.
From its roots as a website with no ads, no business plan and a hacker ethic, Facebook has grown into a company worth $150 billion, with 6,337 employees and a sprawling headquarters in the heart of Silicon Valley. Born in the age of desktop computers, three years before the iPhone's debut, Facebook is now mainly accessed on mobile devices. Many of these mobile users never had a PC.
"People often ask if I always knew that Facebook would become what it is today. No way," Zuckerberg wrote — where else — on his Facebook page Tuesday. "I remember getting pizza with my friends one night in college shortly after opening Facebook. I told them I was excited to help connect our school community, but one day someone needed to connect the whole world."
Facebook has had plenty of stumbles along the way, from privacy concerns to user protests when Facebook introduced new features, not to mention a rocky public stock debut in 2012. Even its origin was the subject of a lawsuit and a Hollywood movie.
So far, though, Facebook has trudged on.
As Facebook enters its second decade, the company faces a new set of challenges in reaching the next billion users, the billion after that, and the one after that, including the majority of the world without Internet access. It must also keep the existing set interested even as younger, hipper rivals emerge and try to lure them away.
There are 1.23 billion Facebook users today, or roughly 17 percent of the world's population. Although that's far from connecting the whole world, Facebook is here to stay. It's reached critical mass.
"One of the things Facebook has been good at is that it's very easy to use and understand," said Paul Levinson, professor of communications and media studies at Fordham University. "It's a much friendlier system than any email system."
As far as birthdays go, Facebook's brought out reflection, nostalgia and lots of memories.
Connie Zong, who signed up for Facebook during her sophomore year at Harvard 10 years ago, remembers when she heard that Zuckerberg was dropping out of Harvard to work on Facebook.
"I remember thinking that guy is making such a big mistake," she said. "He's giving up a really great degree at a great university, and we're never going to hear from him again."