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 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."
Javier Olivan joined Facebook Inc. as vice president of growth and analytics in 2007. It was a different time. Myspace was the dominant online hangout with 200 million members. Facebook had 30 million.
Facebook's user base had been accelerating steadily, Olivan said, as it expanded from Harvard's campus to other colleges, then high schools, and in 2006, anyone over 13. Users in the U.K. and other English-speaking countries then began signing up.
But around 2007, growth plateaued.
"The thinking at the time was (that) we'll never have 100 million users," Olivan said. "That's when the growth team was created."
If Facebook was going to connect the world, as its mission states, it couldn't be an English-only service. So Facebook turned to its users to help translate the site. A Spanish version came in 2008, followed by dozens of others. Growth accelerated again, and volunteer translators are still adding new tongues, whether that's native African languages or pirate slang.
Facebook got its 100 million users by August 2008 and half a billion two years later. By 2012, a billion people were logging in to Facebook at least once a month.
While sharing photos and updates with friends is a universal experience, Facebook is customized depending on where you live. In Japan, for example, users can list their blood type on their profiles, as it's something that would typically come up in conversation when you meet someone — kind of like horoscopes in the U.S.
Beyond language, another hurdle was mobile. The iPhone came along in 2007, and Facebook's iPhone app soon followed. But the app was slow and buggy, fueling concerns that it wouldn't be able to transform into a "mobile-first" company, as it wanted to be. About the time of its initial public offering of stock, potential investors fretted about its ability to make money from mobile ads.