To get started you need an account with an external ID-provider (currently Facebook). The main reason for this, 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 the site switches to an other ID-provider. 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).
How to delete accounts
Video: How to create an account, and then delete the account.