[tallanvor]

Requiring Facebook/Twitter/etc integration - even just for authentication - is a deal breaker no matter what your privacy policy says. I don't trust any company to manage my identity, nor do I want any company trying to do so.

Obviously many people are willing to use their Facebook or other accounts as credentials, but you will lose potential users/customers by making it mandatory. I don't know how many, but it's something you should consider.