This question already has an answer here:
These answers suggest sending a thank-you note to an interviewer is a good idea:
I recently went through an 8 hour all-day interview of nine consecutive interviews all with senior folks. I was interviewed due to an employee referral (who obviously did not take part in the actual interviews).
Would this scale up, and thus should I send nine separate thank-you notes, or should I just stay in contact with the employee who referred me?