[RickS]

FWIW, I think there's a big difference when the directions are reversed (although that may not be rational).

Example: Log into amazon site with facebook credentials? Hell no. Make amazon purchase through facebook chat bot? Possibly.

When it comes to commercial trust, all third parties are not equal.

[tim333]

You don't have the option to log into Amazon with Facebook or any other such service. I think Amazon are quite happy keeping their customer information to themselves.

[cbhl]

Amazon and Facebook actually partnered in Facebook's early days. Purchases on Amazon would automatically show up in users' news feeds. Headlines at the time were full of things such as "Facebook ruins Christmas" and the like.