Why is an external ID-provider used?

An external ID-provider is used, currently Facebook. The reason is that:
  1. They generally "maintain" their registers to prevent fake accounts (users with multiple accounts).
  2. They have more security features (using multifactor authentication etc).
  3. Furthermore, the end-user will have fewer passwords to remember.
Here is a some other ID-providers that are considered.

What data is taken from the ID-provider?
What data is shared with the ID-provider?
What is the idea for the financing?
Possibly advertising.

Possibly donations via patreon.com or similar.

Source code?