FreeFlarum comes with a few extensions to integrate your forum with social logins:
You can enable the ones you like. It is recommended to register an account by using Flarum first so you won't have to deal with password problems later when you decide to use Flarum login instead of social one (when you login by using social media, there is no password, so if you register by using social media, you could have problems with non-existing password later when using Flarum login).
If you use your social media E-Mail when registering an account using the traditional, Flarum way (eg. E-Mail address + Password), that E-Mail will then be picked by all social medias that the E-Mail belongs to and you shouldn't have problems. Real problems occur when you use multiple different E-Mail addresses.
It is always good to register using the "old" way and login using the social logins, just to make your life easier.