More than 350 million active users; Average user spends more than 55 minutes per day on Facebook (http://www.facebook.com/press/info.php?statistics)
(6.7 billion * 24 hours)/(350 million * 55 minutes) = 500
It's unclear how they're measuring time spent on the site, but most of Facebook's statistics seem to be honest (they only count active users, etc).