Christmas Cards are just so damn impersonal. They're like an even less personal precursor to the narcissism of Facebook, which I've been guilty of in the past: "here's how awesome my family is. If you're not us, feel bad about yourself!" I actually considered it relatively harmless until I heard about a neighbor of mine growing up. Her husband is well known and loved, so they get tons of Christmas cards every year. Hundreds. Their son suffered from mental illness and took his own life last year. For her, the act of opening cards was too much and too painful. There were absolutely no personal messages, which is normal, but not one "Sorry this must be tough", etc. Again, not something anyone does. It was all smiling Christmas card photos of happy families all together and they just reminded her of how broken hers is. I'd imagine that happens to a lot of people. They get those cards showing everything all merry and bright with no personalization and it's a reminder of how much worse off they are.
So yeah. freak Christmas cards. At least Facebook doesn't force itself into your mailbox.