Even with just that single example the scaleability issue becomes apparent. I would never want to trust all of my Facebook friends with my computer security. A

Even with just that single example the scaleability issue becomes apparent. I would never want to trust all of my Facebook friends with my computer security. And even less so for my friends' friends, etc. And I certainly could be wrong in my understanding, but I believe all it takes is a single malicious (or pressured) actor to ruin that chain. That seems scary to me.