L.B.
It is so rude for someone to receive a gift and not acknowledge it. About a year ago I mailed a baby gift to my niece and she never acknowledged the gift. I felt a little hurt when I repeatedly saw her posting and mentioning gifts that her friends and closer relatives gave on FB. After about 6 months, I sent her a FB message and asked her if she received it. She responded that she did and that the blanket I sent was the baby's favorite. About a week later, I got a Thank you in the mail.
In this case it must have just slipped her mind. Still, I think that a thank you note should be a priority especially if it is a gift received in the mail and the sender has no way of knowing if the person actually sent it.