I think Thank you notes are still being sent. I know my daughter sent them for her baby shower. Iwrote all my thank you's for my wedding and people kept asking me why they didn't get one... I kept saying "but I maled them!" only to find them YEARS later stamped and rubber-banded together in a stroage box. oops.
I would epxect a Thank You note for a formal occasion (wedding gift, shower gift, graduation, etc.) but I have a coworker who sends a thank you note for EVERYTHING. It gets embarassing actually.... last week I gave a coworker lunch. I had a coupon for buy-one-get-one free. I gave her the free item. A few days later I got a Thank You note in the mail. Seriously? I don't expect a formal thank you for an impromptu gesture.