"As of this morning, the Facebook community is now officially two billion people," founder and chief executive Mark Zuckerberg posted.
The milestone comes just 13 years after the network was founded by Mr Zuckerberg when he was at Harvard.
He famously dropped out of the university after launching the global social-networking website.
The internet giant announced it had one billion monthly users in October 2012, meaning it has doubled the number of its users in just under five years.
The firm's continuing growth will confound critics who have long predicted that the social network's growth would slow down as rivals such as Snapchat stole its users.
Earlier this year, Facebook warned that growth in advertising revenues would slow down.
Nonetheless, Mr Zuckerberg's ambitions remain huge.
He told USA Today the firm had not made "much fanfare" about hitting the two billion figure because "we still haven't connected everyone".
"What we really care about is being able to connect everyone," he said.
The firm's rapid growth has put pressure on its ability to moderate violent and illegal content posted on its site.
The most recent high profile incident involved a man in the US posting a video of himself to the site, showing him shooting and killing an elderly man.
Last month Facebook said it was hiring 3,000 extra people to moderate content on its site.