Handwritten letters are not inherently the superior thank you method.
Wow, according to this link, not only do most employers prefer email (not that surprising) or phone calls (fairly surprising to me) for a thank you, only 38% even think it is appropriate to do a hand written note. That is shocking that more than 60% of employers in this survey actually put some sort of negative connotation / inappropriate label on handwritten notes. This makes me rethink some things. Whether I have been the interviewer or the interviewee I have always sent or expected a handwritten note.
Furthermore, 27% say social media is okay for the thank you (only 11% less than those who approve of the handwritten note) and 10% say a text message is appropriate (that's low, but I am still surprised that it's even that high). I would definitely have a WTF? moment if someone tried to facebook or twitter a thank you note. A linkedin message would even be a little strange to me.
HR managers also were asked, "Which of the following are appropriate methods for job applicants to thank an employer following an interview?" Their responses:
81% Phone call
38% Handwritten note
27% Social media
10% Text message