That's a valid point, but I can't see it being an impossibility that the user will never accidentally stumble sending a http request rather than https. Whether that is user input, or a maliciously placed link.

My understanding is that the HTST header would make this attack less useful. But it's still a concern if you used Private Browsing/Incognito. The initial request will still hit a 301 (vulnerable to interception by MITM). I've just verified this with Facebook.com on my machine (Chrome 26, OSX).

I think it's quite fair to expect a user using this kind of site is likely to use Incognito.

I'm actually kind of surprised as I thought Chrome had a standard list of sites that use https only such as Facebook. (Woah.. seems the preload list is TINY http://www.chromium.org/sts )