So, it turns out that this blog called Read Write Web had a post about people using Facebook login to log in to different sites. (This site also has Facebook login as an option for people to comment on their blog.)
It also turns out that a large number of people in this world only know how to find Facebook by Googling the phrase "facebook login," and that for a while the top Google hit for that phrase was the aforementioned blog post.
This was the resulting comment thread: a whole bunch of angry people asking why the "new" Facebook wouldn't let them see their Farmville, and complaining about the "new" Facebook redesign (ie. the blog they were reading, which they assumed was Facebook). My favourite is the one person who likes the change. And the addition to the blog post that begins with the words: "Dear visitors from Google. This is not Facebook."
And sometimes I wonder why every other storyline on the comic strip Mary Worth is about the dangers of The Internet, and about how The Internet leaves you open to scammers. This is probably why.