Saying Facebook has a hand in the ecommerce market is still very speculative at the moment. That may happen but right now it doesn't even look like they're moving that direction really.
And also, Amazon attracts me definitely for their vast inventory but primarily because of their reviews which falls under the social interaction / peer-peer trust you ascribe to Facebook.
True, and Facebook can't truly leverage any of its "friend-based" data without ugly privacy skirmishes. Because Amazon didn't start out claiming that your opinions (eg. reviews) were private, they have no trust issue to overcome. Trust is the elephant in the room for Facebook.
And also, Amazon attracts me definitely for their vast inventory but primarily because of their reviews which falls under the social interaction / peer-peer trust you ascribe to Facebook.