The demographic that googles "facebook" can easily have different behavior patterns from people who type in the URL.

I'll bet at least some of the difference in facebook search traffic is offset by the rise in popularity of mobile devices. Most people with mobile devices have an app to go to and would be less likely to search 'facebook'.

And don't forget that Firefox sends address bar input that doesn't look like an URL to Google's I Feel Lucky search. That's why a significant part of the Mozilla Foundation's income comes from Google.

I think you'd be surprised at how many hackers enjoy the conveniences of Google search for a known url.