Business Insider unearthed the figure in an SEC filing ahead of Facebook's IPO later this year.
The social media firm recently announced plans to monetise its mobile platform by introducing 'Sponsored Stories' to the newsfeed, a method similar to Twitter's Promoted Tweets platform.
Facebook is also generating 3.2 billion 'Likes' and comments from users each day, 300 million uploaded photos daily and a total of over 125 billion friendships.
The half a billion mobile users milestone is up from 350 million in September 2011.
http://www.mobile-ent.biz/news/read/...e-users/017829