There's a bug in Outlook 2013. See this : http://social.technet.microsoft.com/Forums/fr-FR/d265a5c6-9f99-410a-8862-a374de1b4144/freebusy-not-publishing-with-outlook-2013-without-exchange
This issue has been submitted to the Outlook Team for a fix but we do not have an estimated release date yet. Once I get more information I will post back and update. When using POP accounts the *.vfb file does not get created properly.