to whom are PAN PAN PAN calls addressed?
A PAN call is like a MAYDAY call in that you are broadcasting to whomever may hear it.  However, if you broadcast a PAN call on an active ATC frequency, ATC will naturally respond to it and it's sufficient to just state PANPAN-PANPAN-PANPAN and your registration and any other information and ATC will answer, and everybody else will know to stay off the frequency momentarily.  

You can broadcast a PAN call on any other frequency like a unicom, or 121.5, and whomever is listening in will likely respond.  I was once downed in the bush in a seaplane by engine trouble, becoming stuck on a remote lake, and couldn't raise anybody on the normal unicom channel even if I heard other seaplanes passing in the distance.  I saw a contrail above, and knowing airliners monitor 121.5 over sparsely settled areas, I broadcast a PAN call with my registration on 121.5. To my surprise the airliner heard me and answered.  I was able to get him to contact base on the Unicom frequency and let them know what lake I was on before he was out of range of my radio's transmitter, and was rescued with my two fisherman passengers later that day.  The only other choice was to set off the ELT and hope an airliner or SAR aircraft heard it (this was pre-SARSAT), which would have meant spending the night there at least.