To get started you need an account with an external ID-provider (currently Facebook ). The reason is to ensure the uniqueness of the vendors. (See also FAQ for more reasons)
No data is taken from ID-provider other than name, image and email. However you can change these under →Vendor settings.
No data is shared / no messages sent to the ID-provider.
To ensure your integrity, you can at any time delete your account (all stored data about you), by clicking →User settings→Delete account
Video: How to create an account, play around and then delete the account.
Note, some buttons have moved into different menus etc since the video was recorded. But the login procedure is basically the same.