Making participation in a SE site depend on membership of a third party site is just icky (beyond your OpenID provider of choice, which you can become yourself). Beyond a tacit endorsement of the third party site/service, its more merge conflicts when the SE site has to be updated.
There may be experts who can contribute something who just got fed up with FB and deleted their accounts, I don't see why they should be squelched.
If the powers that be are willing to codify your request, great :) But graceful fall back is needed, and we're again delving into sites running variants of SE software that are sufficiently diverse to present an interesting path to mass upgrade issues.
Additionally, some people might not want that information shown or even stored, for whatever reason.