A yunohost administrator can create a subdomain via the web admin. Next this operation he could be able to set up a yunohost app on this subdomain.
So generally the administrator makes a choice about the subdomain name. If you really want force the subdomain to be named “social”. You could create directly the subdomain with the install script of the yunohost app. But this way is not the standard way, all the other apps let the administrator determine the subdomain name.