I want users to be able to log in (signup) in site with their Facebook or Google plus account. But I didn't find anything for Drupal 8.
Is there any module or solution for doing this job or I should downgrade and use Drupal 7 in our project?
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.)
You can use module Simple OAuth. Some more details about it, from its project page:
... is an implementation of the OAuth 2.0 Authorization Framework: Bearer Token specification.
Using OAuth 2.0 Bearer Token is very easy. See how you can get the basics working in less than a minute!
This project is focused in simplicity of use and flexibility. When deciding which project to use, also consider other projects like OAuth, an OAuth 1 implementation that doesn't rely on you having https in your production server.