I can see some real problems with that method.
I've noticed my use of Facebook has been restricted and constrained by mistakes I made when I first used Friendster and Tribe and Orkut. I see my family and friends making many of the same mistakes I made - playing the competitive "more friends" game and connection to people they've never met or who they really don't want "snooping" on their social life - exes, coworkers, bosses, friends-of-friends…
There's no way I'd want Facebook (or Twitter or Google or Yahoo or Microsoft or … ) being any part of a "web of trust" I was using for privacy/encryption/authentication – partly because there's no doubt they're deeply in bed with the NSA (are you really suggesting Facebook's platform is trustworthy enough to exchange keys?), but at least as much because I can clearly see that most people haven't curated their social networking "connections" with anything like the rigor they might have done if they'd been told up-front that "these connections might be used to authenticate your identity and communication to others (potentially including government, law enforcement, and other legal/contractual entities), and also to authenticate your connections identities and communication to those entities."
Do you _really_ know who all of your Facebook "friends" are? Are you _sure_ the person you think that account represents is actually in control of that account? Even if they are, do you trust them enough to vouch for your identity? Are you sure enough of that trust that they wouldn't "betray" you if the NSA, or a police officer, or their local council's dog-catcher, or your car/health insurance company approached them with either a threat or a handful of cash?
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.
