It's nothing social. It's piggybacking on the authenticity of AngelList and their focus on startups. Who else has as good and reliable of a database? If they opened up signups without some sort of verification, it would dilute the usefulness of the product completely.

You only import from AngelList if you're a company. Otherwise if you're a developer you can either go through Twitter/LinkedIn/Github or create an account via e-mail.