Nothing! And no, you cannot email or facebook or twitter the thank you.
There is no reason not to get that thank you note in the mail - just follow these basic points and you will make a good impression every time. It is not rocket science.
1. Write it in a timely manner.
Why? First, someone went out of their way (some more than others but so what) and sent/gave you a gift or did something nice for you. Even if you don’t like it!
Second, the giver might wonder if you ever got the gift if you never acknowledge it.
2. Address the “Giver” -" Dear So and So."
I recently got a thank you note from someone who addressed the actual note to only part of the family but not all members who sent the gift. I am speaking here of the actual note, not the envelope. Were there too many names to mention in the Dear So and So line? Don’t write Dear Jane and Rick, etc. or "Dear Fam". Abbreviations like this seem impersonal and lazy - not good. How much longer could it possibly take to add the rest of the names?
3. Mention the gift or the event.
This is a very nice and personal touch – if it is money - well, you can refer to the generous gift or what you plan to use it for. Aunt Cori might like to know that the money she sent you is going to be used to take your girlfriend out for dinner or for that iphone you want.
One of the most curious notes I ever received was a beautifully written thank you from a young boy on his personalized stationery. He did not mention my specific gift. This would have actually been impossible for him as I did not yet GIVE it to him (it was specially ordered for him and had yet to arrive). Thus, his thank you, while one of the timeliest notes I have yet to receive, could not possibly have been personal.
4. Length. Short and Sweet.
The note does not have to be long, just thoughtful and prompt. Nothing wrong with short and sweet.
5. Sign off.
Don’t forget to sign your name! (Even if it is on your stationery.)
Of course, personalized stationery is a nice touch, impressive, and convenient – you will have the stash ready and you won't need to go out and buy a generic card. How nice.