Would this have happened in ReadWriteWeb looked more like Daring Fireball?
One of the causes of this kind of mistaken identity problem is that users have become conditioned to a web full of irrelevance and distraction. RWW is typical of many sites: a huge amount of the content loaded for any article is stuff meant to distract you from the article itself. Not just advertising, but crap. How much of that singing, dancing circus is actually necessary? If it isn't necessary, why is it there?
Is it not ridiculous that RWW's message to these confused users is placed in bold black text in the article that they demonstrably did not come there to read and not anywhere amid the colorful parade of nonsense and not anywhere near the text "Sign in with Facebook". It's even below the fold on my system. Do people still want to blame user stupidity alone? Please.
Like all humans, when these users wind up somewhere unfamiliar, they reach for whatever is familiar. In this case, the article has not one, but two Facebook logos above the fold. One in the article text, reading "Facebook", immediately below the word "Facebook" in the headline. Why is this necessary? The other is in the header, and links to RWW on Facebook. But so does the huge Facebook sidebar widget a few scrolls down. Just in terms of pixels, Facebook has already carved out a nice chunk of the page, competitive with that of the body text. And then there's the text itself, the little Facebook logos on all of the user pictures of people who likewise thought this was somehow related to Facebook, and most damning of all, the text "Sign in with Facebook" right above the comment form.
A person who has just Googled for "facebook login" and who just saw a half dozen Facebook logos, and just saw dozens of other people who made the same mistake complaining, who then mistakes that username/password field as meaning "Sign in to Facebook" is not making an epic error in judgement. They are making a simple mistake based on a limited understanding of things they have never had a reason to understand. They aren't stupid, they're human.
I don't really mean to single out RWW, though. This is a web-wide problem. You will never be able to eliminate stupid mistakes or the "better fool", but you can stop gently assisting such confusion and illiteracy by thinking about why what you're doing isn't helping.