I'm 31... and I always want to... but never do.
It tends to be a time / money / futility aspect of it. Time and money is obvious. I have an average of over 100 people that I stay in contact with thoughout the year. Even at only $1.00 per person, that's $100 I can spend on my son/family for presents. But with the costs of postage, plus cards themselves, and photos (even printing them out requires photo paper) it's more like $2-$6 per person.
Futility is that I'm actually in CONTACT with people year round.
Growing up, we didn't have inexpensive phone service (much less "free" internet services like gmail, skype, etc. We pay monthly for the internet but most of the programs we used are paid via ads, not by us). Today I can talk for 2 hours to my friend in Argentina for free (skype), videoconference with another friend in Kosovo (free, skype), Pay 2 cents a minute to talk to family in Italy (because they're older and don't do things like Skype, I can skype my own phone and pay next to nothing to talk to them on their landlines or cell phone), talk with family all over the country (with no extra charge, just my regular cell bill), chat with my brother in London over one of a dozen different services (gmail is what I use).... email nearly anyone for free... and be uploading pictures on any one of a dozen services.
When I was a kid, often the ONLY way we could stay in contact with people was via real, honest to god, mail. Once a year we would get the yearly update with attached photos at christmas time. A few times scattered throughout the year we'd get and send letters.
Now that my son is a child, he can get ahold of ANYONE he wants to (or vice versa, grandparents can actually video conference with him), 24/7 either for free or a pittance. No one is waiting with baited breath to find out how he is, or how our lives have been this year... they know... because no matter how far away they are, they have been a PART of our lives all year long.
I WANT to, because they're a fun festive thing to do, but they aren't necessary anymore... they're an extra. And extras we just can't afford.