Jitsi has never worked properly on Fedora 18, but it was fixed to the point it started up and could make/receive calls, although if you minimized it you could never get it back in Gnome 3.6
However the latest nightly build as broken it totally again.
[user@localhost ~]$ jitsi
/usr/bin/jitsi: line 26: cd: /usr/share/jitsi: No such file or directory
Error: Could not find or load main class net.java.sip.communicator.launcher.SIPCommunicator