To get started you need an account with an external ID-provider (currently Facebook). The main reason is to ensure the uniqueness of respective user. (See also FAQ for more reasons)
The data taken from the ID-provider (Facebook) are name, image and email. Email is used to identify you even if an other ID-provider were to be used. Name and image are used as default values for your convenience (you can change these in the settings).
No data is shared with the ID-provider (and nothing is written to your Facebook flow or anything like that).
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, and then delete the account.
Note! The video show a slightly older version of the software, some buttons have moved into different menus etc. But the login procedure is basically the same.