Is there a way I'm not aware of to derive a private key from a public key? If I only ever give facebook my public key how the hell would they ever get my private key? Are you saying facebook hacks my home desktops to steal private keys?
If you read what denis-The-menace wrote, you'll see Facebook could ask users to give their private key to their (presumably closed-source) client, which could do anything with it. Responding with suspecting them of having some method of deriving the private key, or that uneducated users would really only give Facebook public keys, or Facebook hacking desktops does not address denis-The-menace's actual concern: public-key cryptography is very easy to exploit when the user-base is uneducated in its use, and Facebook offering such a new service to the masses is exactly the path one would follow to inspire users to feel secure all the while sabotaging them.
I am not sure I agree with denis-The-menace, but I wanted to point out that you didn't actually address his actual concern.