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 U.S. 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.
As expected, the longer users are on Facebook, the more "friends" they have on the site. A user who signed up two years ago has an average of 305 friends. Someone who signed up in January 2006, when Facebook had 25 million users, now has nearly 600 friends, on average.
CEO Mark Zuckerberg marked the milestone on his Facebook page, as he has in the past when the site's users hit nice round numbers.
"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, but people are focused on what they're building," he told Matt Lauer during the interview.
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 by shareholders who don't think it's increasing revenue fast enough, especially from its fast-growing mobile user base.
Last month Zuckerberg gave his first interview since Facebook's shaky IPO and since that time he's been working hard to boost confidence among investors, employees and the public.
The 28-year-old executive also continued to reassure that he is the right person to lead Facebook, as some on Wall Street have questioned whether he has the ability to lead a large public company.
"I take this responsibility very seriously," he said.
Facebook Inc.'s stock slipped 9 cents to $21.74 in morning trading. The shares are off 43 percent from its $38 IPO price.