Facebook Login provides a way for users to login to an app using their Facebook account. It also allows users to grant permissions to the app to access their information or perform actions on Facebook on their behalf. There are also identical SDK's available for android and web development.
The 1Password App Extension SDK enables users to access their 1Password logins on an app login page. It also allows for use of the Strong Password Generator to create unique passwords during registration, and save the new Login within 1Password.
Firebase gives you functionality like analytics, databases, messaging and crash reporting so you can move quickly and focus on your users. Firebase is tested and trusted by major companies due to it being run by google.
Auth0 is a platform that offers native or web based app development, server side web applications and , backend and API support. Auth0 gives developers and companies the building blocks they need to secure their applications without having to become security experts through authentication and authorization services.