Facebook says that, for the first time in its history, one billion people - or one in seven people on earth - have logged on in a single day.
In a post on his own Facebook page, the social network's founder, Mark Zuckerberg, said the milestone was passed on Monday.
Facebook has nearly 1.5 billion users who log in at least once a month, but this was the most in a single day.
The company gained its billionth user in October 2012.
Facebook was founded in 2004 by Mr Zuckerberg while he was a student at Harvard.
In his post on Thursday, he predicted that Facebook's reach would continue to grow.
"This was the first time we reached this milestone, and it's just the beginning of connecting the whole world," Mr Zuckerberg wrote.
In July, Facebook claimed that over half of the world's online users visited the site at least once a month.