I have three Outlook users in my office that Internet Free Busy (IFB) does not work for:
1) Outlook 2013 Professional upgraded from Outlook 2010 running on Windows 7 Pro
2) Outlook 2013 Professional freshly installed in a Windows 7 Pro VM
3) Outlook 2013 Professional freshly installed on Windows 8 Pro
Two other users remain on Outlook 2010 and IFB continues to function as expected.
All users are part of a workgroup in which we set up a FreeBusy shared folder on a common network share. Each user's Outlook is connected to a POP3 server.
The three affected users also noted that other settings (e.g. months to publish) in the IFB dialog do not persist (e.g. number of months to publish will change to some random number after closing the dialog, or will always stick at a number the user didn't ever select).