I pick someone at random. (And even then usually only if i'm leveling something and want to get the challenge log bonus for the week.)

There are rare cases where I'll comm someone specific. Usually if someone saves a run. Last person standing at the end of a boss on a near wipe. Or a healer that saves the run with a well timed lb3. Stuff like that. But it's usually on a whim and pretty rare that someone stands out in particular for it.