Normally, iOS shows all of the social service icons, even if one isn’t logged in. If the user clicks the icon, it will ask for them to create an account or log in. Is this possible to replicate with the U3DXT plugin?
No, that is not currently possible with the default native UIActivityViewController, which is what SocialXT.Share() uses. You could however, create your own Unity button for Twitter/Facebook in your app/game, and then call SocialXT.Post(), but make sure to pass in false for the last argument, for when the user has no account set up, it would prompt the user to go to settings.