CEO Mark Zuckerberg announced the number during a live event early this afternoon.
Last week, Zuckerberg hinted at something "awesome" coming today. Early rumors tipped to in-browser video chat powered by Skype. A live event is underway now.
He didn't say when, because Facebook doesn't see the metric as "significant now".
The next five years isn't going to be just about active users, Zuckerberg said. "The rate at which they're sharing more stuff." He said it's growing at an "exponential rate" -- "that's really profound".
Facebook reached the 500 million subscriber threshold a year ago this month, that's up from about 30 million in March 2007 -- less than a year after the social network opened to the public. For the two years earlier, Facebook was closed to all but select participants, mainly college and university students.
Facebook's reach is nothing less than phenomenal. Ninety-two percent of American adults on social networks use Facebook, according to Pew Internet. More than half use the service every day.
The average age of social network service users is 38, with over half 35 or older. Fifty-six percent are female.
"Forty percent of users have friended all of their closest confidants," according to the report. "This is a substantial increase from the 29 percent of users who reported in our 2008 survey that they had friended all of their core confidants."
Facebook friends have been a hot topic this week because of new rival social network, Google+. Developer Mohamed Mansour released a Chrome extension for exporting friends to Google+, which Facebook blocked.