Every account on Memorang has a unique username and email. If you have an existing account with a specific email (e.g. joe.biden@whitehouse.gov), you can login to the same account with Facebook, Google, or Apple as long as the email of that social provider matches your current account. For example, if your Facebook and Google accounts both use the same email, you can use "Continue with Facebook" and "Continue with Google" interchangeably.
If the email is different (e.g. joe.biden@gmail.com vs. joe.biden@whitehouse.gov) then you will end up creating a new account, since these point to different identities.