Updated to the latest Plasma Desktop, on one of my systems (Fedora 23, x86_64), and found that the ‘Favorites’ had all vanished, apart for xterm..    My other x86_64 system updated OK, with all the ‘Favorites’ intact..

Rather than hunt down all the apps and ‘add to favorites’ for each one, I found the old Favorites again, as follows:

Go to ~/.config

Edit  plasma-org.kde.plasma.desktop-appletsrc

Remove the ‘new’ entry marked [Containments][1][Applets][2][Configuration][General]
and find the original ‘Favorites’ settings under [Containments][1][Applets][2][Configuration]

Then rename [Containments][1][Applets][2][Configuration]
to [Containments][1][Applets][2][Configuration][General] and restart the desktop..

The original ‘favorites’ should then appear, and the format is updated in plasma-org.kde.plasma.desktop-appletsrc..

This does not always occur, as my ‘older’ system (with the same software versions etc.) did not suffer from this problem..

Robert Gadsdon.  December 9, 2015.