It's also possible to correlate the shape of the social graph across services to deanonymize users across social networks. So I don't think not proving ownersh

It's also possible to correlate the shape of the social graph across services to deanonymize users across social networks. So I don't think not proving ownership of multiple identities buys much for anonymity. For deanonimization just a reasonable probability is sufficient. But a proof adds more utility to the person proving their identity. You can still have hidden throwaway accounts when needed.