What are requirements to add external identity providers? I want to allow users from Google or Facebook to login to my application.

Azure AD B2C acts as a proxy of other external identity providers. Currently, it supports the following identity providers:

Microsoft - Microsoft personal cloud logins 
Facebook - Facebook logins
LinkedIn - LinkedIn logins 
Amazon - Amazon logins
Weibo - Chinese Weibo logins
QQ - Chinese QQ logins
WeChat - Chinese WeChat logins
Twitter - Twitter logins
GitHub - GitHub developer logins 

Here are the requirements for you connect an external identity provider into your Azure AD B2C directory:

1. Yourself need to have a login account with the provider. For example, your Gmail account is good enough to connect Google as an external identity provider.

2. You need to register the Azure AD B2C proxy to the external identity provider as a client application. See other tutorials on how to do this.

3. Record the "Client ID" and "Client Secret" generated by the external identity provider after the registration is done.

4. Define an Identity Provider entry in your Azure AD B2C account to connect your application to the external identity provider indirectly this the proxy. You need to provide "Client ID" and "Client Secret" obtained from the external identity provider.

The following diagram shows you how Azure AD B2C connects your application to external identity providers:

⇒ Register Azure AD B2C as a Google Application

⇐ Register Azure AD B2C Application

⇑ Azure AD B2C Integration

⇑⇑ OpenID Tutorials