The only way to have done this is by having the user click "continue" or "ignore" or something on an ssl error page. I know from experience that a company full of programmers will happily do that. Only a few percent would go "wait a minute, this is Facebook. That certificate should be valid." Some people here might reply "no way", but HN generally contains the one percent.
Edit: This is almost correct. You can actually prevent being redirected from http to https when typing in "facebook.com" without https:// in front. My bad.
Still though, the attentive user would notice the missing padlock. I check it 3/4 times, and 4/4 times when using a public network. I also refrain from using http sites where I log in (some forums I visit do that). But again, probably less than one percent of the tech people do that.
