Is it rude not to acknowledge e-cards?

I have a close friend who I have sent a few e-cards to over time (a birthday e-card & also an encouragement one). She never acknowledges them in any way. I feel this is rude. What do you all think? I'm not looking for huge accolades, but even a "Hey, got your e-card! Thanks!" would be fine. Now that I think of it, she never acknowledges when I send her a Christmas card or postcards when I am on vacation either. Don't you agree this is rude of her? Should I say something?