When we were kids, my mom my always forced us to write thank-you notes after birthdays and Christmas. Truth be told, it was like pulling teeth around here (as I'm sure it is in many families) to get them done. Finally, she wouldn't let us have the gift (usually money) until we sent the card - hello, motivation!
I'll be honest, though, and admit that I couldn't tell you the last time I wrote a thank-you note, slapped a stamp on it, and put it in the mail. I still think it's important to acknowledge a gift (or special meal or visit) in some way, but I can't help but think that these formal thank-yous are very passé.
In this fast-paced, technology-driven world, I think it's much more reasonable to expect a phone call, e-mail, Facebook message, tweet, text, etc. than a hand-written note in the mail. And I think there are a lot of people who would appreciate the immediacy of these options over waiting a week (or more) for a note to arrive in their mailboxes. Also, odds are much better that I have your e-mail address than your street address.
I'm just being honest here... unless it's for my grandparents, a card is probably not going to happen. (Unless, of course, it's for a formal event, like my wedding. A formal event calls for a more formal thank-you, in my opinion.)
I also have to say that I don't ever expect or keep track of who sends me a thank-you. This may not be a very popular opinion, but to me, expecting a thank you is like expecting to be praised for being nice or thoughtful - and that just seems silly. We should be nice and thoughtful of others because it's the right thing to do, not because we want to be acknowledged for it.