Hi Shek Kan Cheung,
For signing in to ArcGIS Online using a Google or other social media account, users will need to create an account first. That said, you can sign up for an ArcGIS Online account using your third party identities first and then use it to sign in whether in the browser or in ArcGIS Earth. If you are signing up for a trial account, using a username/password is currently the only option. Once that admin account for the trial organization is in place, you can invite other members via using their Google or other supported accounts to join the trial org.
Hope that can clarify the workflow. Please let us know if you have any further questions by commenting below or sending them to ArcGISEarth_Feedback@esri.com.
Thanks,
Joe