More than a billion people now log into Facebook each month to check up on old friends, tag photos of new ones and post about politics, religion, cats or what their kids are doing.
That's double the 500 million it hit in July 2010.
The latest milestone also amounts to nearly half of the world's roughly 2.5 billion Internet users, as measured by the International Telecommunications Union.
So who are these people?
Most of them -- 81 percent -- live outside of the United States and Canada. Many of them log in on mobile devices rather than personal computers, and the company now has 600 million mobile users.
The people joining now are young, with a median age of 22. It was 23 in 2010 and 26 in 2008 and 2007. Most of them are from Brazil, India, Indonesia, Mexico and the United States. They are unlikely to be from China, the world's most populous country and home to its largest Internet population. And millions of them are not actual people. Facebook acknowledged in August that 8.7 percent of its then-955 million users may be duplicate or false accounts. At that rate, as many as 87 million accounts are fake.
Chief executive Mark Zuckerberg marked the milestone on his Facebook page. "If you're reading this: thank you for giving me and my little team the honor of serving you," he wrote. "Helping a billion people connect is amazing, humbling and by far the thing I am most proud of in my life."
But he acknowledged in a "Today" show interview that the company is going through a difficult patch. "We're in a tough cycle now and that doesn't help morale," he said.
The Menlo Park, Calif.-based company's stock never recovered from a botched initial public offering in May, at one point seeing its value slashed in half.