Interesting. I did some playing around with them today; looks like once you have the calling card (and the friend) they each function independently of the other. I can delete calling cards for friends, and I'm still friends, and I can remove a friend, and the calling card stays.

I remember a few months back when Mr. Shang said he'd had to axe his friends list because all the updating of their status was causing him to crash and hang and crash and hang, and the only solution was removing several hundred of them. I wonder if simply removing all the calling cards would have worked instead?