a considerably different Christmas card list each and every year

So, I'm putting together all of our Christmas cards to mail out this year. While I'm thinking I may switch over to e-mail cards sometime in the future, I do love the feel of sending out something tangible that my friend is going to also hold in their hand. It feels real, even though we are all so far away from each other.
As I was going over all my old addresses and sending out the cards I was reminded of the fact that we have gone through another transition- friends we hung out with a ton about 18 months ago are now relegated to our occasional email category. People I didn't even know existed 6 months ago are now buddies we see multiple times a week. I'm realizing with increased awareness that doing the central urban thing means, sadly, that it seems like we are caught in a current of people that we meet, get to know, and then KIT a year or two later. We went to a white elephant party tonight and had a total blast with our current buddies. But I'm looking at them from a new perspective- how many of them are going to be here next year to do the same thing before the current pulls them, or us, away? I've learned from enough cycles through New York and 2 moves in LA that most of us won't be here next year- we will have moved to Vegas or Arizona or Utah or maybe even the Valley where we don't have to deal with schools and tight living quarters for big rent. Doing the central urban thing means that we don't set up traditions, we just live it up the best that we can, which is really fun. It also means I get to introduce friends that I meet and will be moving to the same city, which is a great connection to build for all of us, and it means I get to meet so many wonderful, amazing people with a variety of gifts and experiences to strengthen and support one another, but I do feel a bit sad. I felt a bit sad as I had to cross some names off my address list- people I've lost touched with so much I know they've moved, and I know they have a new address, but I can't easily get in contact with them to find out what it is. I wonder if it will always be this way, and if I might be a little sad at the revolving door nature of our experience, or if I can just embrace this time in our lives and the good things it offers- like a considerably different Christmas card list each and every year. ;)
ps. Is it like this for people that live in non-urban center places too?