Login with Facebook or Google+ account
This specific signup type is called OpenID. And it allows an external login provider to login users on your site.

You can have a look at OpenID Connect module. Which has a beta version for Drupal 8 available, and according to the module page already ships with a Google implementation. 


  OpenID Connect 1.0 is a simple identity layer on top of the OAuth 2.0
  protocol. It allows Clients to verify the identity of the End-User
  based on the authentication performed by an Authorization Server, as
  well as to obtain basic profile information about the End-User in an
  interoperable and REST-like manner.


I am not sure about Facebook, but it might be that is in another contrib module or easy to implement yourself. (I didn't look for it.)