Ask HN: 'Facebook's One-Click-Login' email and security implications

If you dont engage with Facebook for x number of days, Facebook sends an email with one-click-login URL which directly logs you into your FB account.

What do folks feel about the security & technology behind it? Has anyone tried 'one-click' checkouts/payments/account access by just sending custom-URLs? (Amazon's OneClick is done after an authenticated session, not before)

So I assume this is secured by "email-address-authenticated" delivery. As in, if you can open your email/text, then it is you.