Facebook 'has over 600m users'

"Facebook has "600 million+ monthly active users," according to a document Goldman Sachs is showing clients interested in investing in Facebook at a $50 billion valuation.
Goldman, which is itself investing $450 million in Facebook, is in the process of offering $1.5 billion worth of common stock to its richest clients.
Facebook announced it crossed the 500 milliion monthly active user mark back in July. The site's official stats page says there are "more than 500 million active users" on the site. Now we know there are at least 100 million more."

Source:  Business Insider, 5th January 2011

Earlier - the last official update