After years in the customer service industry (bartender then Deskside support), I know nothing gets fixed unless you complain. and nothing that works gets recognized unless you compliment.
A good waiter or waitress finds out from my tip. (up to 25% of tab for excellent service) their manager finds out when I talk to him on the way out.
and this works for bad service as well. I never leave no tip (then you become that ignorant a$$). but bad service will receive a 5% tip and the manager will find out why they only got 5%.
A restaurant or bar owner has so much to do he can't possibly watch all his staff all the time. so a compliment or complaint really helps him.
also my tip is always in cash. I have met some shoddy owners that have skimmed credit card tips.