[facebook] was the most common search query when I was at Google (roughly the period covered by this graph), and [facebook login] would alternate between #2 and #3. Neither of your options seem likely at all to me - over the time I was there, I witnessed people becoming more likely to navigate to sites via Googling them rather than clicking, not less. The trend is even more pronounced with mobile, where basically nobody types in a URL, they use voice search and speak the name of the site.

I would bet on the Facebook mobile app being a major cause, though. Instead of hitting the web version at all, they're opening the mobile app.