Facebook now has more than 500 million users, or about 8% of the world's population.
If it was a country, the on-line social networking site would be the third biggest after China and India.
The site, which launched in 2004, has gained about 100 million new users in the last five months.
Facebook said the number was "an important milestone" and added that it was "humbled and inspired" by the stories of its users, which it is asking people to share on the site.
It says the site is "almost guaranteed" to reach one billion users.
But success has come with problems. Facebook's record in protecting personal data has been criticised, forcing a recent overhaul of its privacy procedures.