I think the problem lies in how those iedentities are established. A prolific Twitter or Facebook account makes a good identity precisely because you have spent time pouring your identity into it. Your family photos, relationships, residence and work history on your Facebook page. Your daily thoughts and actions cataloged on your Twitter feed. While not perfect, they're the best thing we have right now to externally verify that you are who you claim to be. They provide context to the authentication. Any external service that implemented the authentication standard that may arise out of this situation lacks that identifying context and the best case scenarion only serves to prove that the user authenticating is the user who setup the account. Taking away Facebook's and Twitter's context makes spoofing that authentication much easier.
Solving that in a way that doesn't violate the privacy concerns of your users seems like something of a holy grail. Panacea if it exists, but far from demonstrated.
Solving that in a way that doesn't violate the privacy concerns of your users seems like something of a holy grail. Panacea if it exists, but far from demonstrated.