It seems to be pretty random. There are times when, by my own measure, I play terribly (I botch my rotation, waste a dreadwyrm trance or something silly like that) and get 2 comms. Then there are other runs when my rotation is flawless, it's clear I'm far out-DPSing the other DPS in the group, and I raise the healer (I main SMN) and get nothing. /shrug
edit: This is by no means meant to sound salty because I've been known to give comms according to glamour/general hotness of other party members.