A colleague from a certain country with which I'm not too familiar said Americans say "Thanks" ad nauseum for everything. Ditto for "Sorry." I've read studies that say Western women say Thanks and Sorry too much, but not about Americans in general. If anything, people seem to be becoming ruder. For instance, if somebody goes to great lengths to hold a door for you (i.e. he sticks foot in door because his arms are full; waits 30 secs for you to make it to door), isn't a thank you in order?
What about thank yous for emails or PMs? A woman solicited information from me about an academic program I graduated from and I spent 20 mins writing her with all the details to help her in her decision whether to attend. No answer. Same with some PMs for desperate posters on this board.
When a thank you's not forthcoming in person, I make penetrating eye contact and say "You're Welcome." If you are on the non-receipt end, what's your take? Or, if you don't say thanks, "What's your damage, Heather?" (gratuitous 80s reference for those old enough to remember)