Maybe people will think this is heresy, but I rarely give comms I only really give them if I feel like someone made a more obvious above average effort.
Partly it's because I quite frankly simply forget too.

I might give DPS a comm if I see them actually using their abilities instead of being lazy, like MNK using Anatman and charging Chakras and doing all their positionals while using True North and Riddle of Earth accordingly, using Mantra when big damage is coming out from the boss and in big pulls etc.
Same with BLM if I see a BLM with really good uptime who is using their arsenal, basically if you're a DPS and I see that you make an actual effort instead of just doing the bare-minimum.
Or a SAM that is actually playing properly, I do notice.

I almost feel like throwing it at NIN's simply for using Trick Attack on CD because quite frankly it feels like almost no NIN's do it's like they just do the first one and then forget that it even exists.
If I notice that they actually do use it on CD I might throw it at them too or if I see them using their teleport last sec to get extra hits in etc.

Edit: The real answer tho is that if you're a friendly person you're 500 times more likely to get it than if you're a shitty person too.
If someone is just especially nice I might give it to them too.