That said, many of the applications that feature "sign in with Facebook", don't actually need access to my Facebook account. They may just be trying to make it easier for the user to sign in, but they also sometimes abuse that trust, and start posting things on your behalf.

Frankly, I'd much rather generate a random password for randomsiteijustfound.com and not worry too much about their password-hashing policy than trust them to do the right thing with access to my Facebook account.

This is why I like Persona -- if a site really just wants to make it easy for me to sign in, they'll use Persona, and I won't have to worry about them abusing my trust, because I'm not granting them access to anything else. Once I feel like they're trustworthy or useful enough, I can consider granting them OAuth authorizations to my Facebook account.