The company said it passed the milestone on Sept. 14, but only revealed the figure Thursday.
The number of Facebook users has doubled in a little over two years: it reached 500 million monthly active users in July 2010, the company said.
Although Facebook claimed 1 billion "people" were active on the site, user growth would have had to more than double since the last quarter if the figure excluded the duplicate and fake accounts that have plagued the company in recent quarters.
On June 30, the company had 955 million active monthly users, up from 901 million three months earlier, according to a regulatory filing with the U.S. Securities and Exchange Commission. But only 872 million of those belonged to real people: the remaining 83 million, or 8.7 percent of the user base, were duplicate accounts, undesirable accounts used by spammers, or misclassified accounts that really belonged to businesses, according to Facebook's 10-Q filing for the second quarter.
Getting from 872 million real users to 1 billion in 10 weeks would imply a growth rate more than double any quarter in the last two years. Facebook did not immediately respond to a request for clarification of the nature of the accounts counted.
Meanwhile, the new users of the service are getting younger. When Facebook reached half a billion users, the average new user was 23; now, the average user is 22, the company said Thursday.
Some things haven't changed in that time: The top five countries from where users joined the service still include Facebook's home territory, the U.S., along with India, Indonesia, Mexico and Brazil, where Google's first attempt at social networking, Orkut, is also popular.
That list also includes four of the world's five most populous countries (Mexico ranks 11th, according to the CIA World Fact Book), but the most populous country, China, is not a major source of Facebook users because the site is officially banned there.
CEO Mark Zuckerberg described the experience of connecting 1 billion people as "by far the thing I am most proud of in my life" in a note posted to the site.
He's counting on the first billion users to spread the word to achieve his next goal: "Together one day we will be able to connect the rest of the world too."
