Co-founder and chief executive Mark Zuckerberg made the announcement, saying the number is "humbling."
"This morning, there are more than one billion people using Facebook actively each month," he said in a statement.
"Helping a billion people connect is amazing, humbling and by far the thing I am most proud of in my life."
Analysts are divided on whether Facebook can leverage its massive user base for advertising and other revenues and still remain loyal to Zuckerberg's goal of making it a service to connect the world.
Trip Chowdhry at Global Equities Research remained bearish on Facebook, saying the company has failed to come up with a credible expansion plan.
"OK, one billion users, now what? How do you monetize them?" Chowdhry said.
"The problems with Facebook remain the same."
But Lou Kerner of the Social internet Fund says the market is underestimating Facebook's ability to drive revenues higher.
"While Facebook continues to grow its user base, recently surpassing one billion active members, the law of large numbers means that user growth is slowing," he said.
"However, engagement per user continues to grow rapidly, and that's the metric that will increasingly drive revenue."
Facebook said it reached one billion monthly active users on September 14 at 12:45 pm Pacific time. That includes 600 million mobile users.
In its last update in June, Facebook said it had 955 million users including 543 million using mobile devices.
"To be able to come into work every day and build things that help a billion people stay connected with the people they care about every month, that's just unbelievable," Zuckerberg said.
The California company launched a hotly anticipated public offering in May, but has seen its share price tumble amid concerns that it may not be able to grow revenues as users migrate to mobile devices.
Facebook shares, which hit the market at $38 in May, rose slightly during the day but had returned to the opening price of $21.83 by afternoon on the Nasdaq in New York City.
Interviewed on the NBC Today show, Zuckerberg said the company was moving forward despite some rough patches.
"Things go in cycles. We're obviously in a tough cycle now and that doesn't help morale, but at the same time, you know, people here are focused on the things that they're building," he said.
"I mean, you get to build things here that touch a billion people, which is just not something that you can say at almost anywhere else, so I think that's really the thing that motivates people."
Since Facebook's launch, users have produced over 1.13 trillion "likes," some 140 billion friend connections and have uploaded 219 billion photos.
The social network has also seen 17 billion location-tagged posts, including check-ins, and 62.6 million songs which have been played 22 billion times.
The median age of the user is around 22, according to Facebook.
According to the independent website Social Bakers, the largest number of Facebook users are in the United States, over 166 million, followed by Brazil (58 million), India (55 million), Indonesia (47 million) and Mexico (38 million).
Zuckerberg, in an interview with Bloomberg Businessweek released Thursday, said the company was taking the new milestone in stride.
"We don't want to overly celebrate any particular milestone, so what we do is we have hackathons," he said.
"So people will be thinking of ideas and working on prototypes and things that we'll need to do to help connect the next billion people, which I think is pretty cool."
The growth figures have been clouded by concerns about dubious accounts. Facebook's own figures show as many as 83 million may come from dubious sources -- duplicate accounts, pages for pets and those designed to send spam.
Some 8.7% of the accounts may be dodgy, the company said in its quarterly filing with the US Securities and Exchange Commission.