Always found it difficult to say thanks to a particular CC member without looking like I m coming onto them or sounding corny...
But always say thanks on disembarking and if someone/crew was particularly helpful then I would write to airline.
My most recent letters have been combination of both - I ve had problem during flight or related to flight but also the crew have been exceptional in trying to sort problem, so although I ve had reason to complain I ve also had reason to complement the crew... wonder if in this type of situation the compliment is passed on?
PW