I don't do Christmas cards, nor do most of the people I know. I don't do Christmas to begin with, and even if I did, cards just aren't my thing. I don't *mind* getting them, but I have received Christmas cards from people I haven't seen in years and in those case I don't track them down to thank them--I assume since I'm getting one and they barely know me, they're sending out a boatload. It never occurred to me they would want me to do that. If it were someone I saw frequently I'd say something.
But I think it's fine to not send yours, because if they aren't sending out cards, they can't exactly be offended that you don't send to them, right? I have a bit of anxiety myself about getting cards and presents, but I accept them in generous spirit. In most cases the spirit *is* generous and they are happy. If they were upset about my not reciprocating, well, they obviously don't know me that well and haven't been listening to me anyway!