Architecture for merging multiple user accounts together

Question

Okay, I got a website where you can register yourself and login. You can also login with your facebook, twitter or linkedin account.

It is important that users only

Answer 1

Most of the posts are quite old and I guess Google's free Firebase Authentication service wasn't yet around. After verifying with OAuth, you pass the OAuth token to it and get a unique user id which you can store for reference. Supported providers are Google, Facebook, Twitter, GitHub and there is an option to register custom and anonymous providers.