Facebook claims an enormous amount of monthly active users (845 million peeps) and daily active users (483 million peeps). DealBook reports
that these numbers include people who do not visit facebook.com.
DealBook found a definition buried on page 44 of Facebook's prospectus that says Facebook includes users who "share content or activity with his or her Facebook friends or connections via a third-party Web site that is integrated with Facebook" as active users. For example, if someone is on a blog and they like a blog post then Facebook considers them an active user. A comment left on a website through your Facebook login also counts you as an active Facebook user. These people are actively using Facebook, but they are not visiting facebook.com.