In the "old days" (maybe 5 years ago and prior) I would occasionally get free drinks for random reasons that were never anything more than being nice.

Never got a note from an FA though.

I got a business card from a United 757 captain one time, with a little note on it thanking me for flying. When I checked the United forum here, I learned that there were two or three pilots who did this. I still have the card somewhere...flight was in the early 2000's.