Key signing enables keys to be exchanged in a hostile environment. Compare web-of-trust to x.500 directory services, which are dependent on CAs.
It also isn't necessary to use Facebook for key exchange in order to adopt social networking functionality to enhancing a web of trust.
The bottom line is you have to design a secure system to avoid having to trust cloud services. While Facebook may be the poster child for untrustworthyness, you can't trust your own machine in the basement of your house not to get hacked. What you can trust is key signing, because it requires stealing a number of identities all at once.
It also isn't necessary to use Facebook for key exchange in order to adopt social networking functionality to enhancing a web of trust.
The bottom line is you have to design a secure system to avoid having to trust cloud services. While Facebook may be the poster child for untrustworthyness, you can't trust your own machine in the basement of your house not to get hacked. What you can trust is key signing, because it requires stealing a number of identities all at once.