Sending thank you notes is good manners in some countries
In others, the polite thing to do is to say thank you in person. Sending thank you notes is not even taught at all.
Technology does affect how we communicate. Paper and ink are becoming obsolete and more inconvenient than a text message, quick email ,etc., especially when time is very scarce and our cell phones are available to help us make our lives easier.
Something about thank you notes for baby presents- the new parents are super busy with the new baby, healing from birth, etc, to have "send thank you notes" on top of the list.
Send thank you notes involves knowing the address or looking for it, buying the cards and stamps, write the card, drop it in the mail....
It is not as simple as "send thank you notes"
What's more important, knowing your present was appreciated- verbally or via social media- or getting a card?