[by]

That depends on how you define 'active'. A quick Google gives me some Facebook published statistics [1] which say that more than 250 million people log on every day. Goldman Sachs seem [2] to say "600 million+ monthly active users".

[1] http://www.facebook.com/press/info.php?statistics [2] http://www.businessinsider.com/facebook-has-more-than-600-mi...

[jfarmer]

For Facebook "active users" always means "users active within the last 30 days."