On ArtStation, you can sign in with Facebook, as well as email/password. There are, however, a few things you should be mindful of.
I have an existing ArtStation account and want to add a sign in with Facebook account
Do not attempt to sign in with Facebook yet. Instead, go to your settings page and hook up Facebook from this page when you are logged in.
I have an existing ArtStation account, but when I clicked "Sign in with Facebook", it created me a new user. Now I have 2 accounts. Help!
This happens because ArtStation tries to match accounts by email address, and your Facebook and ArtStation accounts do not have the same email address. With mismatching email addresses, there is no way that ArtStation can tell that the Facebook account belongs to a specific account.
To fix this:
I tried hooking up ArtStation with Facebook, but it says that another account is already using this Facebook account.
This happens because at some point, you probably accidentally clicked "sign in with Facebook" and it created a duplicate account. To fix this, you must remove the duplicate account.
I don't have an ArtStation account and want to sign up using Facebook.
Simple, just sign up for a new account and click the Sign in with Facebook button instead of registering with your email/password.