To get started you need an account with an external ID-provider (currently Facebook ). The reason is to ensure the uniqueness of respective user. (See also FAQ for more reasons)
No data is taken from ID-provider other than name, image and email. You don't have to show these to the public if you don't want to (you can change these in the settings).
No data is shared to Facebook and nothing is written to your Facebook flow or anything like that.
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. (of slightly older version of the software)
Note, some buttons have moved into different menus etc since the video was recorded. But the login procedure is basically the same.