I send thank-you notes for gifts or hospitality, a kind gesture and the like. Rarely for dinner, unless it was a special occasion like Easter lunch etc. (I will be sending one out to my Aunt on Monday.) Instead, I follow up with a phone call the next day.
I must admit--I don't bother sending thank-you notes to friends who don't bother themselves! It irritates me that they are so rude. And I am 37 years old. One woman told me she thought that thank-you notes were a generational matter for "old" people. She is 38. I felt like saying that actually, it is a matter of class and upbringing. But I bit my tongue.
I did have a friend who made her own cards for special occasions and would send beautiful thank-you notes for dinner in the mail. They always cheered me up just looking at them on the mantle shelf, they were so pretty!