Facebook's user stats, made official in the IPO filling, are certainly impressive. Though not exactly new or surprising, Facebooks' 845 million monthly active users, 483 million daily active users.
But it turns out that Facebook's definition of an "active" user may not necessarily be your definition of an active user. Facebook counts as an active user anyone that clicks on a like button or interacts in any way with Facebook but doesn't necessarily visit the site.
Anyone that actively "likes" something is probably going to visit the site at least once a month. The bigger problem is with daily active users. It is very likely that someone that likes a video on a site may not visit the site that day.
The issue is, for now, that Facebook only monetizes users on its site, but not across the web.