It sounds like you would benefit from offering a menu of registration choices: - Facebook / Twitter / Google - Email+ other

As with AirBnB registration, you would strongly encourage users to register using a social media auth but allow them to use an email + nothing else if they desire.

You can then apply a simple risk scoring model that says (I'm simplifying here):

* Facebook + 200 friends + > 3 year account age = auto-approve

* Email only = red flag; manual verification

Critically, you don't reveal to red-flagged users that they've been placed in a sandbox.